Note: Descriptions are shown in the official language in which they were submitted.
CA 02421886 2003-03-13
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METHOD OF REMOVING HIGH DENSITY STICKIES FROM SECONDARY
PAPERMAKING FIBERS
Technical Field
The present invention relates generally to papermaking fiber processing and
more particularly to a method useful for removing stickies from secondary or
recycle
pape, pulp by incorporating a hvbrid multistage forward cleaner system with an
integrated flotation cell. The method is particularly effective for removing
stickies
that have already passed through a screening stage.
Background
Processing of papermaking fibeis to remove contaminants is well known in
the art, including the use of forward cleaners and flotation cells. Such
technology is
used, for example, to treat secondary (recycle) fiber sources for re-use in
paper
products such as towel and tissue, paperboard; coated writing and printing
papers and
so forth. Equipment utilized includes screening devices, flotation cells and
the like as
may be seen, for example, in United States Patent Nos. 4,272,315 to
Espenmiller;
4,983,258 to Maxham; 5,240,621 to Elonen'et al.; and 5,693,222 to Galvan et
al.
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Recycling paper into secondary pulp suitable for re-use in high quality
products is a relatively complex; capital intensive undertaking as will be
appreciated
from United States Patent No. 5,587,048 to Streisel et al. The basic cleaning
sequence of the `048 patent is as follows: (1) detrashing - the detrasher
contains 6mm
(1/4 inch) holes and retains large contaminants, such as plastic bags, pieces
of wood,
large staples, pieces of metal and packing tape, detrashing typically takes
place at 3-
5% solids; (2) high-density cleaning - heavy, coarse contaminants, such as
bolts,
staples and rocks are removed, high density cleaning typically takes place at
about 3-
4% solids; (3) primary coarse screening - primary coarse screens contain holes
2-3
mm in size, preferably 2.4 mm, for removing medium-sized contaminants, such as
small fragments of wood, tape and styrofoam, coarse screening at this stage
protects
fine slotted screens downstream from being' overwhelmed by contaminants that
are
large relative to the slot width, `arid results in improvement in quality and
production
rates, coarse screening typicallytakes place at about 2.5-3.5% solids; (4)
secondary
coarse screening - the rejects from the primary coarse screening may be
screened
again using holes of the same size, but at a lower consistency, about 1.5-2.5%
solids;
(5) sand cleaning (centrifugal) - sand cleaning at this stage protects the
fine slotted
screens downstream from excess wear, waste corrugated paperboard contains
relatively large amounts of sand, cleaning ahead of the screen increases the
cost of the
system, and increases the requirements for hydraulic capacity, sand cleaning
typically
takes place at about 1% o solids; (6) screening - fine slotted screens are
used with a
width of 0.008 inch (0.20 mm), rather than 0.012 inch previously used for
corrugated
paperboard, the fine screens remove plastic` slivers, wax and stickie
agglomerates,
screening takes place at less than '1% solids, preferably less than 0.9%; (7)
Lightweight Cleaning (Gyrocleaning) - lightweight cleaning preferentially
removes
materials with a specific gravity below 1.0, such as plastics, waxes and
stickies, not
heretofore removed, lightweight cleaning is performed at about 0.8% solids.
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It should be appreciated from the '048 patent that existing methods for
handling
stickies removal were based on removing light contaminants having a density
generally less
than the fiber being cleaned. Such methods have been found inadequate when a
significant
amount of heavy stickies are present.
SUMMARY OF INVENTION
In the past there were mainly small light weight stickies that managed to get
through
screens, and most of these small light weight stickies were subsequently
removed by the
gyro-cleans. More recently, heavy weight stickies started becoming a problem;
presumably
because some of the new pressure sensitive adhesives tend to form heavy weight
stickies. The
small heavy weight stickies, which managed to get through screens, were also
accepted by
the gyro-cleans or reverse cleaners, but they were subsequently rejected with
a lot of fiber by
the forward cleaners. Since the heavy weight stickies from the forward
cleaners are still
hydrophobic, it is possible to selectively remove them with a flotation cell
after the
hydrophobic particles attach themselves to air bubbles in the flotation cell.
The heavy weight stickies are difficult to remove by flotation if they lose
their
hydrophobic properties during the deinking process (e.g., due to the addition
of dispersing
chemicals) or if the flotation cell is operated inefficiently (e.g., at too
high a consistency or
with insufficient air bubbles or due to inadequate contact between stickies
and air bubbles).
One advantage of,having the flotation cell on the forward cleaner rejects is
that it is
possible to keep the consistency low, since only 10-30% of the total flow is
being treated (the
percentage depends on reject flow amount). If all the stock is treated in a
flotation cell, the
tendency is to raise the consistency from 0.5 - 0.6% to
CA 02421886 2003-03-13
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4:.
1% or higher to keep the size and cost of the equipment down. If the design
consistency is already 1%, the heavy weight stickies removal efficiency
becomes
even worse when the consistency rises above 1% due to production increases. By
installing a flotation cell on the forward cleaner rejects in an existing
process, it is
possible to design the hybrid cleaner flotation cell system at 0.5 - 0.6%
consistency
and obtain improved heavy weight stickies removal efficiency.
The present invention generally includes a method of processing secondary
fiber to remove high density stickies which have a density generally greater
than the
fiber including: (a) processing a fiber feed stream to generate a rejects
stream
enriched in high density stickies and (b) treating the rejects stream enriched
in high
density stickies with a flotation stage to generate an intermediate flotation
purified .
stream with a reduced high density stickies content. Preferably, the feed
stream is
initially processed by way of a centrifugal separation technique, such as
feeding the
stream to a bank of hydrocyclones, which generate a rejects strearn enriched
in high
density stickies. The high densiiy stickies typically have a characteristic
area (that is,
projected maximum cross-sectional area) of less than about 0.5 mm2, and
usually less
than about 0.3 mm2. The treatment by the flotation stage is effective to
remove at
least about 40 percent of the high density stickies present and, in most
cases, at least
about 50 percent. The high density stickies are believed to be derived from
pressure
sensitive adhesives.
In another aspect of the'invention there is provided a method of thin stock
processing secondary fiber to rernove high density stickies having a density
generally
greater than the fiber includes the steps of: (a) processing a feed thin stock
stream at
a consistency of less than about 2.5%, preferably less than about 1%, to
generate a
thin stock accepts stream and a thin stock rejects stream, the thin stock
rejects stream
being enriched in high density stickies; and (b) treating the thin stock
rejects stream
enriched in high density stickies to generate an intermediate flotation
purified stream.
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The present invention provides in still another aspect a hybrid system for
processing papermaking fibers and includes a multistage array of forward
cleaners
coupled with a flotation cell which increases overall efficiency of the
system. In a
typical embodiment, a first rejects aqueous stream from a first stage bank of
5 centrifugal cleaners is treated in a flotation cell before being fed to a
second stage
bank of centrifugal cleaners. The accepts stream of the first stage bank of
centrifugal
cleaners is fed forward as is the accepts stream of the second stage bank of
centrifugal
cleaners. Preferably, the two accepts streams are combined.
One advantage of feeding the second accepts stream forward is that it does not
have to be returned to the first bank of cleaners for re-cleaning. This
reduces the size
of the first bank of cleaners or allows an existing installation to operate at
a lower
consistency. (The cleaners operate more efficiently at a low consistency of
0.5% than
at 0.8 or 1%). Another advantage is that the flotation cell typically operates
at greater
than 60% efficiency on removing hydrophobic contaminants from the first
cleaner
rejects, while another cleaner stage removes less than 50% of the hydrophobic
contaminants. As a result a large quantity of hydrophobic contaminants are
removed
in the flotation stage, which makes the remaining cleaner stages work more
efficiently with less good fiber loss.
As will be appreciated, by one of skill in the art, the size and cost of a
flotation
stage for treating secondary fiberrcan be reduced by up to 75% if it is
installed in
centrifugal cleaner system as compared to a full scale treatment of the stock
by
flotation. The centrifugal cleaner system modeling indicates a 34% reduction
in ink
speck area of total centrifugal cleaner system accepts by removing ink specks
from
the first stage rejects with 80% efficiency in a flotation stage and then
feeding the
flotation accepts forward after centrifugal cleaning of the second stage. (24%
reduction if second stage rejects are treated in a similar manner). The
ability to feed
the centrifugal cleaner rejects forward (after the flotation stage and
additional
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centrifugal cleaning in the next stage) reduces the stock consistency in the
first stage,
thereby improving the efficiency of the first stage. The capacity of the
system is also
increased by feeding the second stage centrifugal cleaner accepts forward. The
other
centrifugal cleaner stages can also be operated more efficiently since more
than 50%
of the ink in the first stage centrifugal cleaner rejects has been removed in
the
flotation stage. When the centrifugal cleamer accepts are thickened in a
press, a large
amount of ink ends up in the pressate. This ink can also be removed by using
the ink-
laden pressate as dilution water for the centrifugal cleaner rejects going to
the
flotation stage.
A conventional centrifugal cleaner system (as shown in Figure 1) normally
consists of several stages, whereby the rejects of each centrifugal cleaner
stage are
diluted for cleaning in the next stage and the centrifugal cleaner accepts are
fed
backwards to the feed of the previous stage."The ink speck removal efficiency
of the
centrifugal cleaner is usually much less than 50% on toner inks in office
waste paper.
As a result the total centrifugal cleaner system ink speck removal efficiency
can drop
to 30% or less on a fu.rnish containing a laige proportion of office waste.
By sending the first or second stage centrifugal cleaner rejects to a
flotation
stage (as shown in Figure 2) it is possible to remove a much higher percentage
of the
ink specks in office waste. (It was possible to obtain 80% removal of ink
specks
during a pilot plant trial with a flotation cell operated on second stage
centrifugal
cleaner rejects.) If the accepts of the flotation cell are cleaned in the next
centrifugal
cleaner stage, the centrifugal cleaner accepts from that stage can then be fed
forward
to the thickener. Sending centrifugal cleaner accepts forward reduces the load
and
improves the efficiency of the previous centrifugal cleaner stage.
The present invention is particularly useful in connection with removing
stickies from a thin stock recycle fiber product stream; likewise, it is
believed pitch
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removal is enhanced. Stickies are generally a diverse mixture of polymeric
organic
materials which can stick on wires, felts or other parts of paper machines, or
show on
the sheet as "dirt spots" or holes. The sources of stickies may be pressure-
sensitive
adhesives, hot melts, waxes, latexes, binders for coatings, wet strength
resins, or any
of a multitude of additives that might be contained in recycled paper. The
term
"pitch" normally refers to deposits composed of organic compounds which are
derived form natural wood extractives, their salts, coating binders, sizing
agents, and
defoaming chemicals existing in the pulp. Although there are some discrete
characteristics, there are common characteristics between stickies and pitch,
such as
hydrophobicity, low surface energy, deformability, tackiness, and the
potential to
cause problems with deposition, quality, and efficiency in the process.
Indeed, it is
possible with the present invention to reduce stickies by 50%, 80% or even
more by
employing a flotation cell in a multistage forward cleaner system as
hereinafter
described in detail. The rejects from the flotation stage are so full of ink,
ash and
stickies that they can be rejected without any further treatment.
As will be appreciated from the discussion which follows, a preferred method
of cleaning recycle pulp includes combining the accepts from a first
centrifugal stage
with the accepts from a second centrifugal stage which is fed with the
flotation-
purified rejects of the first stage. The process is particularly effective for
removing
relatively heavy weight (small size) hydrophobic stickies that have already
passed
through a screening stage. This will increase productivity of a paper machine
utilizing the pulp and decrease paper machine downtime and converting
downtime.
Stickies build up on wires or fabrics and cause holes to form in the sheet
requiring
downtime on the paper machine to remove them. Stickies also build up on doctor
blades in paper machines and get into the dewatering felt and so forth. In
converting,
they can cause problems such as sheets sticking together. They clog emboss
rolls and
interfrere with the proper operation of other rolls, cause holes in the sheet
and so on.
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Solvents are typically required to remove stickies from equipment and this can
lead to
environmental issues.
In recent years, stickies removal from recycle fiber has become more difficult
in many cases. Without intending to be bound by any theory, it is believed
that
stickies generated from waste paper including pressure-sensitive adhesives
become
more flexible at typical operating temperatures (40 C) of screens and thus
tend to
pass through even fme screens.
The method of the present invention has been employed in a commercial
papermill and found to have a dramatic effect on downtime of the mill. Prior
to
installation and employment of'the inventive method of removing contaminants,
the
plant typically experienced about 10 hours of downtime per month due to
stickies.
After employment of the claimed process, the plant has run for eight months
without
a stoppage due to stickies. In preferred embodiments the present invention is
thus
directed to a method of removing stickies from secondary or recycle fiber.
In one preferred mode of practicing the invention there is provided a method
of processing papermaking fibers with a multistage array of forward cleaners
including a plurality of centrifugal cleaners configured to generate accepts
streams
and rejects streams which concentrate hydrophobic contaminants including the
steps
of: (a) feeding a first aqueous feed stream including papermaking fibers to a
first
stage bank of centrifugal cleaners of the multistage array; (b) generating a
first
accepts aqueous stream and a first rejects aqueous stream in the first stage
bank of
centrifugal cleaners, the first aqueous rejects stream being enriched in
hydrophobic
contaminants with respect to the first aqueous feed stream; (c) supplying the
first
-rejects aqueous stream to a flotation stage; (d) treating the first rejects
aqueous stream
in the flotation stage to remove hydrophobic waste from the first aqueous
rejects
stream and produce an intermediate aqueous ' purified feed stream; (e) feeding
the
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aqueous purified intermediate feed stream to a second stage bank of
centrifugal
cleaners of the multistage array, the second centrifugal cleaner being
configured to
generate a second accepts aqueous stream; and (f) combining the first accepts
aqueous stream with the second accepts aqueous stream to form a combined
accepts
stream. A further step involves thickening the combined accepts stream.
Generally,
the process is carried out at a consistency of less than about 1%; typically
at from
about 0.3% to about 0.9%, and preferably at from about 0.4% to about 0.7%. The
multistage array of forward cleaners comprises at least 3 banks of centrifugal
cleaners
in one embodiment.
Hydrophobic contaminants removed from the first aqueous rejects stream by
the flotation stage may include an ink composition, such as a toner ink
composition.
Typically, the hydrophobic contaminants removed from the first aqueous rejects
stream by the flotation stage includes stickies, and may include an ink
composition
and stickies. The process is also believed unexpectedly effective in removing
stickies
derived from pressure sensitive adhesives.
In yet another aspect of the invention, there is provided a method of thin
stock
processing of secondary fiber ta'remove contaminants including the steps of:
(a)
screening a first aqueous stream including secondary papermaking fibers having
a
consistency of less than about 2.5% in a screening device with openings having
a
screening dimension of less than about 10 mils to generate a screened accepts
aqueous stream; (b) feeding the screened accepts aqueous stream to a
multistage array
of cleaners configured to generate centrifugal cleaner accepts streams and
centrifugal
cleaner rejects stream which concentrate heavy hydrophobic contaminants, the
rejects
stream of at least one centrifugal cleaner being fed to another centrifugal
cleaner; and
(c) processing at least one centrifugal cleaner rejects stream of a
centrifugal cleaner of
the multistage array with a flotation stage to remove hydrophobic
contanzinants, the
CA 02421886 2003-03-13
flotation stage thereby generating a flotatiori purified stream having a
reduced
hydrophobic contaminants content.
Unless otherwise indicated, terminology appearing herein is given its ordinary
5 meaning; %, percent or the like refers, for example, to weight percent and
"consistency" refers to weight percent fiber or solids as that term is used in
papermaking. "Mils " refers to thousandths of an inch. The banks of
centrifugal
cleaners are typically hydrocyclone type cleaners.
10 Brief Description of Drawings
The invention is described in detail below with reference to numerous
examples and the appended Figures wherein like numbers designate similar parts
througr-ut and wherein:
Figure 1 is a schematic of a conventional multistage forward centrifugal
cleaner system wherein each ~~ik of cleaners are designated by a conical
element;
Figure 2 is a schematic diagram of a hybrid multistage forward
cleaner/flotation apparatus and process of the present invention, wherein a
flotation
stage is provided to treat the second stage rejects stream;
Figure 3 is a schematic diagram of a hybrid multistage forward
cleaner/flotation apparatus and process of the present invention wherein a
flotation
stage is provided to treat the first stage rejects stream;
Figure 4 is a schematic diagram of a hybrid multistage forward
cleaner/flotation apparatus and process of the present invention wherein a
flotation
stage is provided to treat the first stage rejects and third stage accepts;
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Figure 5 is a schematic diagram illustrating an apparatus and process of the
present invention wherein the hybrid system has dual forward cleaner banks in
series
and the rejects stream from both of the forward cleaner banks are provided to
a
flotation cell;
Figure 6 is a side broken away view of a screen containing a slotted basket;
and
Figure 7 is a plot of residual ink concentration versus location in the pulp
cleaning system.
Detailed Description
The invention is described in detail below for purposes of illustration and
exemplification only. Such explanation of particular embodiments in no way
limits
the scope of the invention which is defined in the appended claims. Referring
to
Figure 1, there is shown a coriventional forward cleaner system 10 of the type
employed at a paper mill, for instance, as part of the cleaning process for
processing
secondary pulp into paper products. System 10 has -five stages 12, 14, 16, 18
and 20
of banks of centrifugal cleaners interconnected in the manner shown. Such
connections may include suitable piping, mixing tanks, holding vessels and the
like
(not shown) as may be convenient for operating the system. Pulp is fed at low
consistency to the system at 22 to the first bank of cleaners 12 through inlet
24 and
centrifugally treated in the first stage by a bank of hydrocyclones, for
example, such
that the accepts are fed forward at 26 to a thickener (or another cleaning
device) at 28
whereas the rejects, concentrating the heavy, hydrophobic waste in the system
are fed
to second stage 14 at 28 for further treatment in a second stage made up of a
second
bank of centrifugal cleaners 14. Diluent water is added to the rejects stream
from the
first stage as indicated at 30 in an amount suitable for the particular system
or
operating conditions. Stream 28 (first stage rejects) is thus fed to the
second stage
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cleaners whereupon bank 14 of cleaners generates an accepts stream 32 and a
rejects
stream 34. Stream 32 is a recycled to the feed 22 and makes up a portion of
the
material fed to. the first stage bank of cleaners 12. The first bank of
cleaners may be
made up of 50 or more hydrocyclones depending on capacity and performance
desired. Subsequent stages will each contain fewer cleaners than the previous
stage
depending upon the amount of rejects, until the final stage contains less than
10
cleaners.
Stream 34 is again enriched with respect to heavy components (with respect to
stream 32) and is fed to the third stage 16 bank of cleaners for further
processing.
Diluent water may again be added at 36 if so desired to stream 34. Stage 16
generates
another accepts stream 38 which is fed back to the second stage (stream 28)
and
another rejects stream 40 enriched in heavy hydrophobic components.
In like fashion, stream 40 is fed to the fourth stage 18 bank of cleaners at
42
where diluent water may again be added. Tfie fourth stage generates another
accepts
stream 44 and another rejects stream 46. These streams have the
rejects/accepts
characteristics noted above.
Stream 46 is fed to ye't'another stage 20 of forward cleaners at 48 wherein
stream 46 is divided into an accepts stream 50 and a rejects stream 52 as
indicated on
the diagram. Accepts stream 50 is recycled to the fourth stage as shown and
rejects
stream 52 is discarded or further processed if so desired. There is thus
described a
conventional forward cleaner system utilizing centrifugal cleaners in
cascaded/refluxing fashion to concentrate the waste material and purify the
pulp
which is fed forward at a papermaking process to a thickening device or a
cleaning
device such as screens or a reverse cleaner.
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In accordance with the present invention, a flotation stage is advantageously
integrated into a multistage forward cleaner system to remove hydrophobic
material
and increase the cleaning efficiency. Flotation utilizes the phenomenon that
the
minerals which are present in the ground ore can partially be wetted, i.e.,
they are
hydrophilic, while other parts of the minerals are hydrophobic. Hydrophobic
particles have a clear affinity to air. Accordingly, fmely distributed air is
introduced
into the solid-water-mixture so that the air will attach to the hydrophobic
particles
causing them to rise to the surface of the mixture or suspension. The
hydrophobic
particles, such as valuable minerals or the above-mentioned contaminants
present in.
repulped stock suspensions, collect as froth at the surface of the suspension
and are
skimmed off with a suitable means such as a paddle or weir. The hydrophilic
particles of the ore or stock suspension remain in the flotation vat. It is
also possible
to separate two or more useful minerals selectively by the flotation method,
for
example, in the separa.tion of sulfidic lead/zinc ores. For controlling the
surface
properties of the minerals small amounts of additives of chemical agents are
introduced such as, for exampie, foaming agents which will help to stabilize
the air
bubbles, so-called collecting agents which actually cause the hydrophobic
effect and
prepare the mineral particles for'''attachment to the air bubbles, and
floating agents
which temporarily impart hydrophilic properties to the hydrophobic minerals
and
later return the hydrophobic properties for selective flotation, as mentioned
above.
The latter are generally inorganic compounds, mostly salts, while the
collectors are
mostly synthetic organic compounds, and the foaming agents are oily or soapy
chemicals such as fatty acid soap.
The apparatus of the present invention may utilize a variety of readily
available components. The centrifugal cleaners, for example, are available
from
Ahistrom (Noormarkku, Finhind) or Ceileco (Model 270 series) (Lawrenceville,
Georgia, USA) and are arranged in banks as shown in Figures 2-5. The flotation
stage, which may be multiple cells, are likewise readily available from Comer
SpA
CA 02421886 2008-01-18
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(Vicenza, Italy). Comer CybercelOO models FCB 1, FCB3 and FCB4 are suitable as
discussed
further herein.
There is illustrated in Figure 2 an apparatus 100 and method in accordance
with the
present invention. Apparatus 100 operates similarly to apparatus 10 in Figure
1. Like parts
are given like numbers for purposes of brevity and only differences noted from
the discussion
above. The system 100 of Figure 2 operates as described in connection with
system 10 of
Figure 1 and is so numbered in the drawing except that system 100 has a
flotation stage 75
for treating the rejects stream 34 of second stage cleaner 14. Diluent water
may be added at
36 as before, and hereafter, stream 34 is treated in the flotation stage to
remove hydrophobic
material. The accepts from the flotation stage, that is purified as shown by
removing
hydrophobic waste from stream 34, is then fed in stream 34' to third stage
cleaner 16. Instead
of refluxing the accepts from the third stage back to the second stage, the
accepts material is
fed forward in a product stream 26' for downstream processing. The hydrophobic
rejects (31')
from flotation stage (75) are removed from system 100.
In Figure 3 there is illustrated another apparatus 200 and method of the
present
invention. Here again similar functioning parts are numbered as in Figures 1
and 2, the
discussion of which is incorporated by reference here. Apparatus 200 of Figure
3 differs from
apparatus 10 of Figure 1 in that a flotation stage 75 is added to treat the
first stage rejects
stream 28 to remove hydrophilic waste to produce an intermediate purified
stream 28' which
is fed to the second stage bank of cleaners 14. Bank 14 generates a purified
accepts stream 32'
which is fed forward to the thickening or other device 28 along with stream
26. The
hydrophobic rejects (21') from flotation stage (75) are removed from system
200.
In Figures 4 and 5 there are illustrated alternate embodiments of the present
invention.
Like components are numbered as in Figures 1-3 above. In the apparatus 300 of
Figure 4,
there is provided a flotation cell 75 which treats rejects stream 28 from the
first centrifugal
cleaning stage along with accepts stream 38' from the third centrifugal
cleaning stage. Stream
38' is combined with rejects stream 28 and fed to the flotation stage where
hydrophobic
material is removed and an intermediate purified stream 28' is produced.
Stream 28' is fed to
the second stage 14 of centrifugal cleaners. The accepts stream from stage 14
is fed forward
as stream 32" and combined with stream 26 in thickening device 28. The
hydrophobic.rejects
(21') from flotation stage (75) are removed from system 300.
CA 02421886 2008-01-18
Apparatus 400 of Figure 5 resembles apparatus 200 of Figure 3 except that
there is
provided a preliminary stage 12' of centrifugal cleaners, the accepts stream
26" of which is
utilized as the feed to stage 12. Rejects stream 28" of stage 12' is combined
with rejects
stream 28 of stage 12 and fed to flotation stage 75. Accepts stream 32' of the
second stage
5 cleaners is fed forward with accepts stream 26 of stage 12. The hydrophobic
rejects (21')
from flotation stage (75) are removed from system 400.
EXAMPLES
Pilot plant trials showed that flotation cells such as the Comer Cybercel can
10 successfully deink secondary centrifugal cleaner rejects, with better
results obtained if the
consistency is kept close to 0.6%. Consistency refers to weight percent fiber
or associated
solids such as ash unless the context indicates otherwise. Results on 42%
office waste (Grade
A) and 100% office waste (Grade B) are shown in Table 1.
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Table 1: Pilot Plant Results for Brightness Gain, Dirt + Ash Removal
Efficiency
on Grades A and B at Halsey and Results Used in Simulation Models
Grade: A B Model
Consistency: 0.69% 0.90% 0.62%
Brightness Gain: 18.5% 5.3%
Dirt Removal: 77-89% 65-87% 80%
Ash removal: 63% 64% 64%
A simulation model was used to calculate the impact of a Comer Cybercel
flotation cell to deink forward cleaner rejects on solids loss, ash removal
and on
removal efficiency of mid-dirt (>150 microns) from a 1s` washer to the deinked
pulp
(while running grade B at 336 tpd at the 1' washer):
,.;
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Table 2: Imuact of Flotation Cell on Solids Loss, Ash Loss, and Mid-dirt
Removal Efficiency
(according to the Simulation Model for 6 different configurations on Grade B)
Example Solids loss Ash loss Mid-dirt Eff.
1 No Flotation cell 8.9 tpd 0.8 tpd 96.1%
2 Flotation cell on 2nd
stage Rejects 2.7 tpd 0.9 tpd 97.0%
3 Flotation cell on
lst stage Rejects -6.7 tpd 1.9 tpd 97.4%
4 As3with50%efin
1" stage 6.7 tpd 1.9 tpd 97.7%
5 Flotation cell on lst
stage Rejects +
3~d stage accepts, 44%
eff. in 1s` stage 8.9 tpd 1.9 tpd 97.7%
6 Flotation cell on two 1s`
stages 11.8 tpd 2.8 tpd 98.5%
The following indicators were used to evaluate the performance of the pilot
plant:
- feed consistency.
- brightness gain of handsheets from accepts compared to feed.
- Dirt removal efficiency of small dirt (<150 . microns), mid-dirt
(>150 microns) and large dirt (>200 microns).
- Ash removal efficiency.
The results in Table 3 below for examples 7-14 (duplicate runs) show that
even at 0.90% feed.consistency it was possible to obtain 5.3% points
brightness gain,
CA 02421886 2003-03-13
18
73% mid-dirt removal efficiency and 64% ash removal on Grade B. Operating the
flotation cell at 0.69% consistency on Grade A, it was possible to obtain 8.1%
points
brightness gain, 79% mid-dirt removal efficiency and63% ash removal.
CA 02421886 2003-03-13
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CA 02421886 2003-03-13
The effect of incorporating a flotation stage in accordance with the present
invention into a multistage forward cleaner system was evaluated with a
computer
model with respect to the systems illustrated in Figures 1-5. Results are
summarized
in the tables below. DIP refers to deinked pulp and DRE refers to dirt removal
5 efficiency.
CA 02421886 2003-03-13
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CA 02421886 2003-03-13
26
The process of the present invention is particularly useful in connection with
thin stock processing of recycle fiber, whereiiz the aqueous stream has a
consistency
of less than about 1% during such processing. Thin. stock processing is
employed in
connection with commercial recycling operations, following pulping, thick
stock
cleaning and washing prior to thickening and bleaching, for example. In a
preferred
thin stock process in accordance with the invention, the thin stock is
screened in a
screening device with a screening dimension of less than about 10 mils to
generate a
screened accepts aqueous stream which, in turn, is fed to a hybrid sytem such
as
shown in Figure 4, for example. The screening dimension of the screening
device is
the slot width of a slotted screen basket, or could be the hole diameter of an
alternate
screening device.
Slotted screening devices are preferred and are well known. There is shown
in Figure 6 such a slotted screening device 500 provided with a feed port 510,
a
screen 520 provided with a plurality of elongated slots such as slots 530, a
rejects
outlet 540 as well as an accepts outlet 550: A feed stream is fed at 510 while
the
rejects stream is withdrawn at` S40 and the screened accepts aqueous stream
560 exits
outlet 550. Accepts stream 566 may then be fed forward to a first stage bank
of
centrifugal cleaners for further dilution and processing as described above.
Slotted screens having a slot width of 8 mils or less may be employed. In
commercial processes, fme slotted screens of 6 mils are frequently employed.
A commercial operation utilizing thin'stock processing as part of its
secondary
fiber processing was operated with a hybrid system as shown and described in
connection with Figure 4 above. The multistage array of forward cleaners was
fed
with thin stock which had been screened with 6 mil slotted screens prior to
being fed
to the forward cleaners. The particular arrangement included in sequence fine
slotted
screens, gyrocleans followed by the forward cleaner/flotation cell system. The
CA 02421886 2003-03-13
27
flotation cell employed was a Comer-CybercellTM device which is preferably
operated
without diffuser plates. The system was installed along with expansion of a
disk
thickener downstream of thin stock processing of the fiber. As a result of
this project
the cleaners started performing better (improved dirt removal efficiency) and
the
hybrid cleaner-flotation cell removed approximately 80% of the dirt, 63% of
the
stickies and 53% of the ash in the Comer feed with a brightness increase of
2.4%
points. Process mid dirt removal efficiency increased 2.4% (from 96.9% to
98.3%)
when running mixed office waste ("MOW") recycle fiber at 540 ton per day (tpd)
input rate. The paper machines have run without stickies problems for 8 months
since the Comer cell came on line.
The new treatment protocol operated well on a furnish containing 1.00%
mixed office waste (MOW) as shown in Table 9, which compares mid dirt removal
efficiency (MDRE)>0.02-0.5 mrn2 before and after Comer flotation cell start-
up:
Table 9: Mid Dirt 12ea;noval Efficiency Before and After Start-up of
Hybrid Cleaner - Comer Flotation Cell on a Furnish Containing 100 /
Mixed Office Waste (MOW)
MidlAirt Removal Efficiency of 1I?irt > 0.02 - 0.5nun "
Time Process Cleaner C mer- Thickener D'asgerger Disperger
Period Cleaner - .y9IP
Before 96.4% 45% 53 /0 - 21% 74% 70%
Hybrid clnr
After 97.7% 50% 79% 1:3 / 78% 73%
Hybrid
The Effective Residual Ink Concentration (ERTC) also improved throughout
the whole deinking system as can be seen in Figure 7. ERIC levels in the
deinked
pulp dropped from 76 ppm without the inventive thin stock cleaning method to
21
ppm with the hybrid fiber when running MOW fiber at 365 tpd.
CA 02421886 2003-03-13
28
The performance of the hybrid cleaner - flotation cell is summarized in Table
10. It shows 2.4% points brightness increase, 82% total dirt removal
efficiency
(TDRE) and 53% ash reduction across the combination. The quality of the 2 d
stage
cleaner accepts was even better than the first stage cleaner accepts.
Table 10: Hybrid Cleaner - Flotation CeIY Results Operating On
1St Stage Cleaner Rejects
YJnit ~eed 13irigh#'0ess Uir,t Rtemovaal Effi;cienc ; Ash
p eratir~n Cons A~h 13r.~ t~aan Srra,p ; 1V~DRE T~~' I2emm.oval
Comer 0.65% 2.0% 70.9% 2.0 % pts 78.8% 64.3% 71.0% 50%
St.2 Cleaner 0.58% 1.0% 72.9% 0.4 % pts 47.2% 51.2% 49.7% 10%
-Acce ts
Camei-Cl1~t~ 0.49%0 0.9% 73.3% 2.4 % ts 85.3% 79.1 % 82 0 70 53%
*Br. Is brightness of feed and accepts, MDRE is mid dirt removal efficiency
and
TDRE is total dirt removal efficiency.
In the plant, the number of stickies in the deinked pulp are counted 3 times
per
shift by screening a 150 gram sample of deinked pulp on a flat screen with
0.006 inch
slots. The count for MOW based fiber averaged 3.3 stickies per 150 grams
before
installation of the hybrid system and improved to - 1.3 stickies per 150
grarris after
implementation of the process.
The area of stickies retained on a Pulmac screen with 4 mil slots was also
measured for selected samples. The uncompressed stickies are then counted
using a
microscope equipped with a grid to estimate the size of the stickies. Two sets
of
samples were obtained at 4 locations in the overall pulp-cleaning process at a
first
date, prior to installation and operation of the hybrid cleaner system (Data
Set A), at
121ocations at a second date, also prior to installation of'the hybrid cleaner
system
(Data Set B) and again at 8 locations in the process at a third date while the
hybrid
system shown in Figure 4 was operating (Data Set C). The average results of 20
CA 02421886 2003-03-13
29
gram stock samples for each location are shown in Table 11. The improvement in
total stickies removal efficiency from 95.0% to 98.5% is attributed in part to
the
improved operation of the hybrid forward cleaning system over forward cleaners
alone.
Table 11: Comparative Stickies Removal
(Small stickies =<0.28 mm2; Large stickies = 0.28 - 1.47 mm2; X-Large stickies
=> 1.47 rnm)
Proces5 Location and I'talffiac Stickies (mm2r400 granis) '`otalRernoval Data
Set Small Large %aLat e. Tatai Efficiency
Data Seà A;
High Density Cleaner 72 219 119 409
ls` Washer - out 76.9 51.3 10 138 15i washer -DIP = 85.3%
Dis er er - in 49.1 0 0 49
Deinked Pulp 20.3 0 0 20 HDCI-DIP = 95.0%
Data Set B
15` Washer - out 64.0 13.3 0 77 1s` washer -DIP = 91.0%
Fine Slotted Screens - out 50.8 3.1 0 54
St.l Cleaner - in 42.9 0.5 0 43
St.1 Cleaner - out 36.9 2.8 0 40
St.2 Cleaner - out 43.6 0 0 44
Dis er er - in 48.9 2.7 0 52
Dis er er - out 31.9 0 0 32
Deinked Pulp 6.8 0 0 7
Data Set C
High Density Cleaner 102 168 37 306
1s3 Washer - out 54.7 10.9 0 66 1S` washer -DIP = 93.0%
Fine Slotted Screens - out 53.1 0 0 53
Comer cell - Feed 48.8 0 0 49 Comer in-out = 62%
Comer Cell - Acce ts 18.1 0.6 0 19
Dis er er - in 35.9 0 0 36 Disp. in-out = 34%
Dis er er - out 21.6 0 0 22
Deinked Pulp 4.6 0 0 5 HDCI-DIP = 98.5%
It can be seen from Table 11 that the Comer cell was particularly effective in
removing small stickies, removing over 60 percent of the stickies in the feed
to the
flotation cell.
CA 02421886 2003-03-13
While the invention has been illustrated in connection with numerous
embodiments, modifications to those embodiments within the spirit and scope of
the
present invention, set forth in the appended claims, will be readily apparent
to those
of skill in the art.
5
