Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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REDUCING CHEMICAL TRANSFER BETWEEN TREATMENT STAGES
BACKGROUND OF THE INVENTION
1. Field of the Invention
.
Washing cellulosic wood pulp.
2. Review of the Prior Art
There is a great amount of prior art describing
bleaching at low consistencies. None discloses isolating
or substantially isolating a stage. Patents describing
low consistency ozone stages are Kempf, et al. U.S. Patent
No. 4,080,249 and Eckert U.S~ Patent No. 4,119,486.
An article describing various consistencies
is Osawa and Schuerch "The Action of Gaseous Reagents
on Cellulosic Materials 1. Ozonization and Reduction of
Unbleached Kraft Pulp," TAPPI February 1963, Vol. 46,
No. 2, pp. 79-84.
Counterflow washing is disclosed in Rapson U.S.
Patent No. 3,698,995, issued October 17, 1972; in Hisey
"Countercurrent Washing in Multi-Stage Bleach Plant Opera~
tion at Brown Company," American Paper Industry, September
1969, pp. 43-45; and in Histed and Nicoller "Water Reuse
and Recycle in the DCEDED Bleach Sequence," and "Water
Reuse and Recycle in the CDEHDED Bleach Sequence, n pp.
133-140 and 167-170, respectively of the preprints of
the CPPA/TAPPI International Pulp Bleaching Conference
given in Vancouver, British Columbia June 3-7, 1973.
The three articles do disclose a split washing stream
to a washer, but none of these articles disclose isolating
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or substantially isolating a washing stage.
There have been installations, usually chlorine wash-
ers, in which the washing fluid applied to the mat has been
split into two different fluids.
BRIEF SUMMARY OF THE INVENTION
The flow of washing fluid onto a washer is split,
allowing the liquid leaving the washer with the pulp mat to
be of the type used to treat the pulp in the stage following
the washer, and the liquid leaving the washer as filtrate to
be of the type used to treat the pulp in the stage preceding
the washer. As an example, pulp is treated in an alkaline
medium, washed and then treated in an acidic medium. The
alkaline liquid with the pulp slurry would leave the washer
as filtrate and any alkaline washing fluid used initially to
wash this pulp would also leave as filtrate. The final
washing of the mat on the washer would be with an acidic
fluid which would displace the alkaline liquid in the pulp
mat and would leave the washer as the liquid in the mat.
Fresh water may be used in place of the alkaline wash or
between the alkaline wash and the acid wash, and a portion
of -the fresh wa~er may also leave with the mat. A chemical
treatment stage may be isolated by using this system in the
washer preceding and succeeding the treatment stage.
STATEMENT OF THE INVENTION
The invention as claimed herein is a process of re-
ducing substantially the chemical transfer between pulp
treatment stages comprising
forming a first pulp mat from pulp fibers and a
first liquid phase,
applying a first fluid to the first pulp mat,
then applying a second fluid to the first pulp mat,
23~
removing a first filtrate from the first pulp mat,
retaining within the first pulp mat a second liquid
phase,
the maximum amount of the second fluid applied to the
first pulp mat being substantially equal to the second li-
quid phase,
treating the pulp fibers with a chemical,
forming a second pulp mat from the pulp fibers and a
third liquid phase, ~ -
applying a third fluid to the second pulp mat,
then applying a fourth fluid to the second pulp mat,
retaining within the second pulp mat a fourth liquid
phase,
the third and fourth fluids applied to the second
pulp mat being substantially equal to the fourth liquid
phase, and
reusing at least a portion of the second filtrate
as the second fluid.
The invention as claimed herein is furthermore a
process of reducing substantially the chemical transfer be-
tween pulp treatment steps comprising
forming a first pulp mat from pulp fibers and a
first liquid phase,
applying a first fluid to the first pulp mat,
then applying a second fluid to the first pulp mat,
removing a first filtrate from the first pulp mat,
retaining within the first pulp mat a second liquid
phase,
the maximum amount of the second fluid applied to
the first mat being substantially equal to the second li-
quid phase,
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treating the pulp fibers with a chemical,
forming a second pulp mat from the pulp :ibers and a
third liquid phase,
applying a third fluid to the second pulp mat,
then applying a fourth fluid to the second pulp mat,
removing a second filtrate from the second pulp mat,
retaining within the second pulp mat a fourth liquid
phase,
removing a portion of the second filtrate from the
system,
the amount of the third fluid applied to the mat
being substantially equal to the sum of the removed portion
and the fourth liquid phase, less the amount of the fourth
fluid, and is at least equal to the removed portion, and
reusing at least a portion of the second filtrate
as the second fluid.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic of a system isolating a wash-
ing stage.
Fig. 2 is another schematic of a modification of the
system shown in Fig. 1.
Fig. 3 is a schematic of another modification
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of the system shown in Fig. 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The following definitions will be used in this
application.
Pulping is the changing of wood chips or other
wood particulate matter to fibrous form. Chemical pulping
requires cooking of the chips in solution with a chemical,
~nd includes partial removal of the coloring matter such
as lignin associated with the wood.
Bleaching is the treatmment of cellulosic fibers
to remove or alter the coloring matter associated with
the fibers to allow the fiber to reflect white light more
truly.
Consistency is the amount of fiber in a slurry,
expressed as a percentage o~ the total weight of the oven
dry fiber and the solvent in the slurry, usually water.
The consistency of the pulp will depend upon
the type of dewatering equipment used. The following
definitions are based on those found in Rydholm Pulpin~
Processes, Interscience Publishers, 1965, pages 862-863
and TAPPI Monograph No. 27, "The Bleaching of Pulp," Rapson
editor, The Technical Association of Pulp and Paper Industry,
1963, pages 186-187.
Low consistency is from 0-6%, usually between
3 and 5%. It is a suspension that is pumpable in an ordinary
centrifugal pump and is obtainable using deckers and filters
without press rolls.
Medium consistency is betw~en 6 and 20%. However,
15~ is a dividing point within the medium-consistency
range. ~elow 15%, the consistency can be obtained by
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filters. Above 15%, press rolls are needed for dewatering.
Rydholm states that the usual range for medium consistency
is 10-18%, while Rapson states it is 9-15%. The slurry
is pumpable by special machinery even though it is still
a coherent liquid phase at higher temperature and some
compression. The consistency of a slurry from a washer,
either brownstock washer or a bleaching stage washer,
is 9-13%. ~;
High consistency i5 from 20-40%. Rydholm states
that the usual range is 25-35% and Rapson states that
the range is from 20-35%. This consistency is obtainable
only by presses. The liquid phase is completely absorbed
by the fibers. The pulp is nonpumpable but for very short
distances.
A neutral fluid is one that does not alter to
any great extent the acidity or alkalinity of a fluid
or liquid with which it is mixed.
The purpose of the present system is to isolate
a treatment stage from surrounding treatment stages.
To illustrate this, the isolation of an ozone treatment
stage is shown in Fig. 1. There are a number of reasons
for isolating this stage.
Firstly, the ozone treatment is at an acidic
pH. Usually the treatment s~age before and after the
ozone treatment stage would be at an alkaline pH. Conse-
quently, the chemical usage would be lowered by isolating
the stage so that additional chemicals would not be used
to neutralize the chemicals from a preceding stage.
Secondly, the pulp in the ozone stage would
usually be at a low consistency. Although the pulp may
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be at any consistency during ozone treatment, it is preferred
that the pulp be at a consistency of 0.01% to 4.9%. The
most effective consistency is considered to be in the
range of 0.01% to around 0.7%, and preferably around 0.37~.
In this discussion, a consistency of 0.01~ to around 0.7%
will be used.
Thirdly, the stage operates better if some alcohol
is present. The cost of operation is reduced if the alcohol
is recycled and reused. Isolation of the stage allows
this.
Fourthly, the stage would normally be at a different
temperature than the stages on both sides of it. Cost
would also be reduced if the temperature of liquid within
the system does not have to be raised and lowered a number
of times. Again, isolation of this stage allows this.
An ozone stage 10 is shown in conjunction with
stages before and after it. ~o specific type of chemical
is indicated for these latter stages.
In this figure, we will first follow the pulp
through the system and then follow the wash water through
the system to show how the wash water used in the ozone
stage is reused and isolated from the rest of the system.
The pulp slurry 12 enters the vat 20 of washer
21 and the pulp fibers are picked up on the drum 22, carried
past washer heads which spray fluid, usually water or ;~
weak filtrate, on the mat to displace the liquid in the
mat with new liquid, is dewatered by vacuum and exits
as pulp 23.
Each of the washers in this system operates
in the same way. They are vacuum drum washers in which
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a vacuum drum 22 rotates through the vat 20. The drum
is covered by a filter cloth. During its rotation through
the pulp slurry in the vat, a vacuum pulls the fibers
onto the filter cloth and the liquid in the vat through
the fibers and the filter cloth into the internal piping
of the drum. The liquid or filtrate is carried through
a pipe central of the drum to an external pipe and into
a storage or seal tank that both holds the filtrate and
maintains the vacuum within the drum.
The consistency of the pulp mat will stay substan-
tially constant during its travel on the drum after leaving
the vat. As much liquid will be removed from the pulp
mat by vacuum as there is washing fluid added to the mat.
This removed liquid is also carried into the internal
piping system of the drum. It is assumed that the washing
fluid will displace the liquid in the mat, although in
actual practice there will be some mixing of liquid in
the mat with the washing fluid and not a complete displace-
ment. The consistency o the slurry entering the vat
120 is usually 1 to 1-1/2% and the consistency of the
pulp 23 leaving the drum is usually 9 to 15%. The fiber
mat is removed from the drum by scrapers, wires, or other
means. These are then cleaned of residual fibers by a
clean-up ~asher 24. A fluid washer is shown, although
this clean-up may also be done with air.
The pulp mat 23 is then carried into a stage
27 where it is bleached vr extracted with appropriate
chemicals. The chemicals may be added to the mat 23 on
the washer or in a later mixer. The pulp is usually diluted,
heated and stored during this treatment. The treated
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pulp slurry 28 is then carried to the vat 30 of washer
31. Prior to entering the vat, it is again diluted to
a consistency of 1 to 1-1/2%. The dilution usually occurs
in storage and between storage and the vat. The operation
of washer 31 is similar to washer 21. The drum is 32,
the exiting pulp is 33, and the clean-up washer is 34.
The pulp 33 is again at a consistency of 9
to 15% and must be reduced to a consistency of 0.01% to
around 0.7% before the ozone treatment. It enters the
mixer 35 where it is mixed with a great quantity of water
to reduce its consistency to the appropriate amount.
This pulp slurry 36 then passes to ozone reactor 37 where
the pulp is treated with the ozone. In these low consis-
tency ranges, the ozone would be mixed with the pulp,
using either a mixing energy of 0.02 to 0.2 hp per cubic
foot of gassed slurry, or a superficial velocity of ozone
and carrier gas of 200 to 3,800 feet per hour. The super-
ficial velocity is the average lineal speed of the gas
through the reactor. The ozone would be present in the
carrier gas entering the reactor in an amount equal to
0.05 to 23~ of the weight of the carrier gas, preferably
0.05 to 6~. A water-soluble alcohol, preferably butanol,
would also be added. The alcohol should be maintained
per liter of the liquid phase in a range of 0.0000001
to 0.03 moles. The preferable range is 0.0001 to 0.0027
moles and the optimum range is 0.01 to 0.005 moles. These
ranges do not depend on the consistency of the pulp but
only on the amount of liquid present. The amount of alcohol
added to maintain this level will depend on the amount
of filtrate recycled. The treated pulp 38 then enters
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the vat 40 of washer 41 and is taken by the drum 42 by
the washer heads and exits as washed pulp 43. It is again
at a consistency of 9 to 15~. The clean-up washer is
44.
Pulp 43 is treated in another stage 47 and the
treated pulp 48 is carried to the vat 50 of washer 51.
It is again diluted to 1 to 1-1/2% prior to entering the
vat. The drum of this washer is 52, the exiting pulp ~;
is 53, and the clean-up washer is 54.
The wash water and filtrate in the system is
flowed countercurrently so it may be reused within the
system. It is also flowed in a manner that isolates the
wash water used in stages 27 and 47 from that used in
the ozone stage 37. It is assumed that stages 27 and
47 are similar so that their filtrates may be combined.
This is done by using two sets of washing heads on washers
31 and 41 so that the filtrate from washer 51 may be returned
to that stage or flowed into stage 27.
Fresh process water from line 60 flo~s both
into washer heads 61 and into clean-up washer 54 and even-
tually through the internal piping of drum 52 and the
external filtrate line 62 into seal tank or storage tank
63. The filtrate in the seal tank may be used for several
purposes. It may be used to dilute the pulp 48 entering
the vat 50. Line 64 and pump 65 are for this purpose.
It may be used to dilute the pulp in stage 47. Line 6~
and pump 69 are for this purpose. It may be used to wash
the pulp mat in a preceding stage. Line 70 and pump 71
are for this purpose. It may become effluent. Line 72
is for this purpose.
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The filtrate in line 70 is split. Part goes
through line 93 to a set of washer heads 94 on filter
drum 42. The filtrate is sprayed on the mat shortly before
the pulp 43 leaves the mat. This filtrate or washing
fluid will enter the pulp mat and an equal amount of liquid
will be removed from the mat as filtrate through the internal
piping of drum 42. However, a large proportion of the
filtrate from washer heads 94 will remain with the mat
and be carried with the mat 43 back into the bleach system
47. Consequently, the major portion of the filtrate from
washer 41 will be from the preceding ozone stage, and
a major portion of the filtrate from bleach stage 47 will
not be filtrate from washer 41, but will be returned to
bleach stage 47.
The rest of the filtrate from line 70 will be
carried by line 113 to either washer 31 or 21.
Whether the filtrate in line 113 will be used
as wash water on washer 31 will depend upon the amount
of contact that can be allowed between the ozone stage
filtrate and the filtrate from stage 47. If there should
be no contact, then the filtrate fro~l line 113 will go
to washer 21, and fresh process water from line 100 will
be used in washer 31.
In either case, the washing fluid will pass
through line 100 to washer heads 101 and clean-up washer
-~ 34. The washing fluid passing through washer hea~s 101
will enter the pulp mat and a substantially equal amount
of liquid will be removed from the pulp mat and be carried
into the internal piping system of drum 32 and leave as
filtrate through external line 102 into seal tank 103.
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The filtrate from seal tank 103 will be used in the same
manner as the filtrate from seal tank 63. Line 104 and
pump 105 will carry it to pulp 28 to dill~te the pulp.
Line 108 and pump 109 will carry it into bleaching stage
127 to dilute the pulp. Line 110 and pump 111 will carry
it to washer 21 to wash the pulp. Line 112 will remove
it as effluent.
The filtrate in line 113 will also be carried ;
to washer heads 121 and clean-up washer 24 on washer 21,
and be removed either as filtrate through line 122 to
seal tank 123 or as liquid with the pulp 23 into the bleach-
ing stage 27. The filtrate from washer ~1 will also consist ~`
of the liquid entering with the pulp slurry and the liquid
removed from the pulp mat while it is on drum 22. From
seal tank 123, the filtrate will be carried through line
124 by pump 125 to be used to dilute the pulp, through
line 130 by pump 131 to be used elsewhere in the process,
or through line 132 as effluent.
In washer 81 of the ozone stage, the pulp mat
is first washed with fresh water. The water is addedthrough line 80 to washer head 81. The water is also
supplied to clean-up washer 84. The second washing fluid
is supplied through washer heads 94. During the application
of washing fluid to the pulp mat by washer heads 81 and
94, there will be a removal of substantially equal amount
of liquid from the pulp mat. This will be removed as ;~
filtrate through line 82. In addition, a substantial
portion of the liquid entering with the pulp slurry will
be removed as filtrate also. This filtrate goes into
seal tank 83. From there, the filtrate is carried by
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line 84 and pump 85 through a heat exchanger 86 to remove
excess heat from the system. Although temperature does
not have an effect on the reaction, it is normal to operate
ozone systems at temperatures under 50C. It is necessary
to cool the system to operate at these temperatures.
The heat exchanger 86 may be used as a heat source for
other streams in the system. For example, if stage 47
requires high temperatures, then heat exchanger 86 may
be used to heat the fresh process water passing through
line 60.
The filtrate in line 84 is split into two portions.
The major portion passes through line 88 into mixer 35
where the pulp slurry is diluted from a consistency of
9 to 15% to 0.01% to around 0.7~. The remainder o~ the
- filtrate from line 84 i~ carried by line 90 to washer
heads 114, the second set of washer heads on washer 31.
The filtrate or washing fluid is added to the pu~p mat`~
and a substantially equal of liquid is removed from the
pulp mat as filtrate from washer 31~ Consequentlyr a
major portion of the filtrate from washer 31 will be from
the preceding stage 27 or the stage 47, and a major portion
` of the filtrate from the ozone stage will not leave washer
31 as filtrate 31 but will be returned to ozone stage
37 with the pulp mat. The remainder of the filtrate may
be removed as effluent through line 92. This amount will
equal the amount being added through line 80.
Alcohol may be added to the ozone stage at two
places, either directly into the pulp slurry 36 at 115
or into the recycled filtrate in line 86 at 116. Most
of the alcohol remaining in the system will be reused
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so only enough will be added to maintain the alcohol in
the system at the chosen level. The amount of alcohol
added to the system will usually be less than 10~ of the
total alcohol in the system, and may well be less than
5% of the total.
The filtrate would also be recycled within the
stage and isolated from the surrounding stages. The purpose
is to retain the great amount of liquid, water, being
used within the stage, and to reduce the volume of effluent
that must be treated before discharge. The recycle and
isolation of the filtrate within the stage also minimizes
pH adjustment. The pH of an ozone stage is acidic - 1
to 7. The pH of the stages before and after the ozone
stage, stages 27 and 47, will normally be alkaline - 7
to 14. By isolating the ozone stage, it is possible to
reduce the amount of alkaline and acidic material required
for pH adjustment because the alkaline filtrate from stage
47 does not have to be rendered acidic before being used
to wash the pulp mat on washer 41, and the acidic filtrate
from the ozone stage does not have to be rendered alkaline
before being used to wash the pulp mat on washer 31.
These reasons would argue for the isolation of any ozone
stage operated at low consistency.
This isolation is accomplished by the amount
of washing fluid applied to the mat at the washers before
and after the stage, and by the method of applying the
washing fluid. The consistency of the pulp in the mat
on drum 32 will usually be between 9 and 15~. For example,
at a 12-1/2% consi~tency the pulp mat will contain 7 tons
of water for each ton o~ pulp, and at 10% consistency
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it will contain 9 tons of water for each ton of pulp.
The amount of washing fluid applied by washer heads 101
and 114 should at least equal the amount of liquid in
the pulp mat so that an amount of liquid equal to liquid
originally in the pulp mat will be removed from the mat.
If the washing fluid supplied by washer heads 101 is not
,~ neutral, then there is a second requirement. The amount
of wash water applied by washer heads 114 should equal
the amount of water in the mat so that an amount of liquid
substantially equal to the amount of liquid in the mat
prior to washer heads 114 will be removed from the mat. ` ,
This same flow pattern also occurs at washer
41. Again, the consistency of the pulp in the mat on
drum 42 will be 9 to 15%. The excess water in pulp slurry
38 due to the low consistency will not be carried by the
mat across drum 42 but will be drawn directly into the
drum 42 from the vat 40 and be discharged through line
82. An amount o~ liquid in the mat substantially equal
to the amount of washing fluid added b"v washer heads 81
2Q and 94 will be removed from the mat and discharged as
filtrate. The amount of washing fluid added by washer
heads 81 and 94 should equal the amount of liquid within
the mat.
Washers 31 and 41 show effluent lines 112 and
92 respectively. If a portion of the filtrate is removed ~`
as effluent, then an equal amount of liquid must be added
as washing fluid at the washer. This is supplied through
the first set of washer heads, washer heads 101 in washer
31 and washer heads 81 in washer 41.
The various lines bringing the process chemicals
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to the system are in the upper portion of the figure.
Line 140 carries process water to lines 60, 80 100 and
120. Line 141 carries chemicals to line 150 for use in
sl:age 47 and line 142 carries chemicals to line 151 for
use in stage 27. If the chemicals are the same, then
the same line would supply both stages. Line 143 brings
the ozone to line 152 for use in ozone stage 37 and line
144 brings the alcohol to line 153 for use in the ozone
stage.
At one extreme, there could be the possibility
of at least two complete changes of liquid in the pulp
mat on drums 32 and 42. In this system, the amount of
washing fluids added by the first set of heads 101 and
81 would equal or exceed the amount of liquid in the mat
and the amount of washing fluid added by washer heads
114 and 94 would equal the amount of liquid in the mat -
leaving the washer. At the other extreme is the possibility
of one complete change of liquid in the pulp mat on drums
32 and 42. In this instance, the washing fluid from washer
heads 101 and 81 would be neutral, and the amount of washing
fluid added by washer heads 101 and 114 would equal the
amount of liquid in the pulp mat leaving the washer and
the amount of washing fluid added by washer heads 81 and
94 would equal the amount of liquid in the pulp mat leaving
the washer.
There are several possible modifications to
this process and these are illustrated in Fig. 2. First,
a pair of washers may be used in place of a single washer,
as shown by the washers 201 and 211 which substitute for
the washer 31 in Fig. 1. Second, there is some chemical
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transfer because this is a total counterflow system with
effluent being removed from the first washer only.
To simplify this discussion, it should be remem-
bered that the amount of liquid in a pulp mat on a filter
drum will remain substantially constant so that the amount
of washing fluid being added to a pulp mat by a washer
head will substantially equal the amount of liquid being
removed from the pulp mat as filtrate. It should also
be remembered that pulp slurry will normally enter the
vat of a washer at a consistency of about 1 to 1-1/2~
and the pulp mat will leave the washer at a consistency
of around 9 to 15%.
We will also assume in this system that stages
188 and 228 are alkaline and the ozone stage 216 is acid,
and that the ozone stage is bein~ operated at a consistency
of 0.01% to around 0.7%.
In this system, a pulp slurry 170 enters the
vat 180 of washer 181 and is carried by drum 182 past
washer heads 321 and the pulp mat 183 is taken from the
drum 182. The clean-up washer is 184. Prior to leaving
the washer drum, the pulp mat is treated with sodium hydrox-
ide at 185 in order to adjust the pH to one appropriate
for the subsequent treatment. The mat 183 is then carried
to treatment 188. In this treatment, it may be heated
with steam to an appropriate temperature for the treatment,
diluted with filtrate to an appropriate consistency for
the treatment, mixed with the chemicals and stored for
a time appropriate to the treatment.
Following the treatment, the pulp slurry 189
is carried to screens 190. Prior to the screening step,
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the slurry is diluted to 1-2% consistency. The screens
remove the larger fiber bundles and knots at 191. The
screened slurry 192 is then carried to the vat 200 of
washer 201. The drum 202 of the washer 201 carries the
pulp mat by washer heads 301 and the pulp mat 203 is removed
from the drum. The clean-up washer is 204.
The pulp enters the tank 210 of the second washer
211 in this series. The drum 212 of the washer then carries
the pulp mat by two sets of washer heads. The clean-up
washer is 214. Before the pulp mat 213 leaves the drum,
it is treated with acid at 215 in order to adjust the
pH of the mat for the ozone treatment. The pulp 213 is j`
then diluted to a consistency of 0.01~ to around 7% in
mixer 216, and the low-consistency pulp slurry 217 treated -
with ozone at 218. The treated pulp 219 enters the vat ;~
220 of washer 221 and the drum 222 carries the pulp mat
by a split series of washer heads and the mat 223 is carried
from the drum. The clean-up washer is 224. Prior to
leaving the drum the mat 223 is treated with alkali at
225 to adjust the pH.
The mat 223 is carried to treatment 228. Again,
the temperature of the mat may be raised, the consistency
of the pulp may be lowered, and the treated pulp may be
stored for an appropriate period of time. The treated
pulp slurry 22~ is diluted and carried to the vat 230
of the final washer 231. The drum 232 carries the pulp
by washer head 241 and the pulp leaves as pulp mat 233.
Again, the clean-up washer is 234.
The alkali added at 185 and 225 may be in an
amount sufficient for an extraction stage which would
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be in excess of the usual pH adjustment. In this case,
the sodium hydroxide normally used would be from 1/2 to
5% based on the oven dry weight of the pulp.
The filtrate flows counter to the flow of the
pulp through the system. Fresh process water through
line 240 is sprayed on the pulp at 241. The filtrate
from washer 231 exits through line 242 into seal tank
243 and then is split~ Part of the filtrate is used to
dilute the pulp slurry coming into vat 230. This part
is carried through line 244 by pump 245. Part of the
filtrate is used to wash the pulp mat on washers 221 and
201. This part is carried through line 250 by pump 251.
The filtrate in line 250 is also split, with
part being used as washing fluid on washer 221 through
line 253 and the second set of washer heads 254, and part
being used on washer 201 through lines 293 and 300 and
washer heads 301. Fresh water is also applied to the
pulp mat on drum 222 through line 260 and the first set
of washer heads 261.
The filtrate from washer 221 passes through
filtrate line 262 into seal tank 263, and then is carried
by line 264 and pump 265 through heat exchanger 266.
The heat exchanger 266 would be used to heat an incoming
water stream such as that in line 240.
The filtrate in line 264 is split, with the
major portion going through line 268 to mixer 216 and
a small portion being carried through line 270 to the
second set of washer heads 274 on washer 211. Fresh water
is also applied to the mat through line 280 and the first
set of washer heads 281.
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The filtrate leaves washer 211 through line
282 into seal tank 283, and from there is split, one part
diluting the pulp 203 entering the washer vat 210. This
filtrate is carried through line 284 by pump 285. The
remainder of the filtrate is carried through line 290
by pump 291 to washer heads 301 on washer 201. It is
combined with the filtrate from lines 293 and 300.
The filtrate from washer 201 is carried through
line 302 to seal tank 303. This filtrate is used to dilute
the pulp slurry entering the vat 200. This is done through
line 304 with pump 305. It is also used to dilute the
pulp slurry 189 entering the screens 190. This is through
line 308 and pump 309. The remainder is supplied to washer
heads 321 on washer 181. It is carried through line 310
by pump 311 to line 320.
The filtrate from washer 181 is carried through
line 322 to seal tank 323 and is there used both to dilute
the pulp 170 entering the vat 180 through line 324 and
pump 325 and carried to effluent treatment through line
332.
The lines bringing chemicals to this system
are shown in the upper portion of the drawing. Line 340
brings fresh process water to line 240, 26~ and 280. :
Line 341 brings acid to line 215. Line 342 brings alkali
to lines 185 and 225. Line 343 carries chemicals to line ~
344 for stage 228. Line 345 carries chemicals to line .
346 for treatment stage 188. I.ine 347 carries o~one to
line 348 for use in treatment stage 218 and line 349 carries
alcohol to line 350 for addition to the pulp mat 217 at
351, or to the dilution water in line 268 at 352.
18
~ ~232~9 12 188
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The precise amounts of fresh water will depend
on the specific pulp mill configuration. However, there
are generalizations that can be made. The fresh water
is divided approximately into three equal amounts to be
added in lines 240, 260 and 280. The greater the amount
of fresh water added, the less solids will be contained
in the recycling slurry. Approximately all of the filtrate
from line 250 will be applied at washer 221 and only a
minute amount will be carried to washer 201. Because
of the amounts of water being used, usually the water
added at washer heads 241 will not equal the amount of
the liquid in the mat on drum 232, so some of the chemical
will be carried out with the mat. The amount of washing
li~uid added at washer 221 through washer heads 261 and
254 will equal or exceed the liquid in the pulp mat, and
the washing liquid added at washers 181, 201 and 211 will
normally exceed the liquid in the pulp mat.
As an example, in a system in which the pulp
leaving each of the washers is at 12% consistency, the
amount of fresh water added per metric ton of pulp by
washer head 241 would be 4 metric tons, and by washer
heads 261 and 281, 3 metric tons. The amount of filtrate
from washer 231 would be 8 metric tons per metric ton
of pulp, and of this 4 would be applied by washer heads
254 and 4 would be applied by washer heads 301. The amount
of filtrate applied by washer heads 274 would be 7 metric
tons per metric ton of pulp. The amount of washing fluid
applied by washer head 301 would be 10.7 metric tons per
metric ton of pulp.
Fig. 3 illustrates anothe~ us~ for the split
lg
~ 2~ 12 18~
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stream on the washer. Again the statements about the
consistencies of the pulp slurry entering the washer,
the consistency of the pulp mat on the washer drum, and
the consistency of the pulp slurry leaving the washer
drum apply. It should also be remembered that the amount
of liquid removed from the mat at the washer would be
substantially equal to the amount of washing fluid added
t~ the pulp.
In this modification, the conditions change
between the stages. These conditions could be pH, tempera-
ture or chemical constitution. For example, the stages
would alternate between an acid and an alkaline pH. There
might be an attempt to reduce chemical usage by counter-
flowing filtrate back to a stage having similar conditions.
The alkaline filtrate would be used to wash the pulp at
a station which would allow it to 10w to an alkaline
stage, and the acidic filtrate would be used to wash the
pulp at a station which would allow it to flow to an acidic
stage.
2~ The apparatus is similar to those described
before, so they will not be described in great detail
here. The incoming pulp is shown by reference number
400. The washer 410 has a vat 411, a drum 412, a first
series of washer heads 413, a second series of washer
heads 414, and a chemical addition 41~. The exiting pulp
is 416, and the clean-up washer is 417. The ~iltrate
exits through line 418 to the seal tank 419. The filtrate
is carried by line 420 and pump 421 to dilute pulp 40n
and by line 426 to effluent treatment. A pulp treatment
stage is denoted by 427. The treatment chemical is carried
~ 2~ 12 188
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to the treatment stage by line 428. The treated pulp
is 429.
In some treatment systems, all of the chemical
used is added at the washer and additional chemical wo~ld
not be added after the washer. For example, in most extrac-
tion systems, the alkali is added at the washer through
line 415 and additional chemical would not be added later.
A treatment stage may include a steam mixer
which adds steam to both lower the consistency of the
pulp and increase its temperature. The alkali or other
chemical may be added at the mixer also. The stage could
al~o include a chemical mixer for mixing an appropriate
chemical with the pulp. These chemicals would include
chlorine, chlorine dioxide, hypochlorite, oxygen, ozone,
peroxide, and various additives. The stage could include
a storage vessel. Filtrate from the following washer
would be used to dilute the pulp as it exits the storage
vessel and before it enters the washer. The pulp may
also be diluted in the mixer.
The other washers have reference numerals similar
to those ~Ised with washer 410. In washer 430, the vat
is 431, the drum is 432, the first set of washer heads
is 433, the second set of washer heads is 434, the chemical
addition line is 435, the exiting pulp is 436, the clean-up
washer is 437, the filtrate line is 438, the seal tank
is 439, the pulp dilution filtrate line is 440 and its
pump is 441, the effluent line is 446, the treatment stage
is 447, the chemical line to that stage is 448, and the
treated pulp is 449. Two other lines are included. rrhe
filtrate used to dilute the pulp in treatment stage 427
~3~ 12 188
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is carried by line 442 and pump 443. The f.iltrate used
to wash pulp on washer 410 is carried ~y line 444 and
pump 445.
In washer 450 the vat is 451, the drum is 452,
the first set of washer heads is 453, the second set of
washer heads is 454, the chemical addition line is 455,
the exiting pulp is 456, the clean-up washer is 457, the
filtrate line is 458, the seal tank is 459, the pulp dilu-
tion line is 460 and the pump is 461, the treatment stage
dilution line is 462 and the pump is 463, the counterflow
wash line is 464 and its pump is 465, the treatment stage
is 467, the chemical to the stage is 468, and the treated
pulp is ~6~.
In washer 470, the vat is 471, the drum is 472,
the first set of washer heads is 473, the second set of
washer heads is 474, the chemical addition is 475, the
exiting pulp is 476, the clean-up washer is 477, the filtrate
line is 478, the seal tank is 479, the pulp dilution line
is 480 and its pump is 481, the treatment stage dilution
line is 482 and its pump is 483, the counterflow wash
line is 484 and its pump is 485, the treatment stage is
487, the treatment stage chemical line is 488, and the
treated pulp is 489.
The washer 490 has vat 491, drum 492, washer
heads 493, exiting pulp 496, clean-up washer 4g7, filtrate
line 498, seal tank 499, pulp dilution line 500 and pump
501, treatment stage dilution line 502 and pump 503, and
the counterflow wash line 504 and pump 505.
In this example, the pulp 400 is acidic, the
treatment in stages 427 and 467 is at an alkaline pH,
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and the treatment in stages 447 and 487 is at an acidic
pH. To maintain these pH's, the chemical added at lines
415 and 455 is alkaline and that added at lines 435 and
475 is acidic. The purpose is to counterflow the acidic
filtrate back to the acidic treatment stages and the alka-
line filtrate back to the alkaline treatment stages so
that less chemical will be required for pH adjustment
between stages.
Fresh water is used only in the last two washers - ~ i
the last alkaline washer and the last acid washer.
Line 510 carries fresh water to line 511 of
washer 490 and line 512 of washer 570. The fresh water
in line 511 is divided between washer heads 493 and clean-up
washer 497. In normal circumstances, the amount of water
added by washer heads 493 is less than the liquid in the
pulp mat. Consequently, some of the acidic liquid in
the pulp mat will remain with the mat 496 as it leaves
the washer 490. The liquid removed as filtrate through
line 498 will be used to dilute the pulp entering the
vat 491 and in the treatment stage 487. It is also used
as the acidic wash water in the preceding washers. The
filtrate carried through line 504 is split, part going
through the second set of washer heads 474 of washer 470 ;
and the rest going through line 513 to the other washers.
The line 513 carries the acidic filtrate to
line 514 on washer 450 to be used in the first set of
washer heads 453 and on the clean-up washer 457; to the
second set of washer heads 434 on washer 430; and to line
515 on washer 410 to be used in the first set of washer
heads 413 and clean-up washer 417. The acidic filtrate
23
: ~Z32~ 125188
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from washer 400 is returned to line 513 through line 458.
It is also used as dilution water for both pulp 449 and
treatment stage 447.
Line 512 carries fresh water to washer heads
473 and clean-up washer 477 on washer 470. The alkaline
filtrate from washer 470 is used to dilute the pulp 469
and the treatment stage 467. It is also used to wash
the pulp in the preceding washers. The alkaline filtrate
carried through line 484 to the second set of washer heads
454 on washer 450; and through line 516 which carries
the alkaline filtrate to line 517 and the first set o~
washer heads 433 and the clean-up washers 437 on washer
430, and to the second set of washer heads 414 on washer
410.
The lines bringing chemical to the system are
shown in the upper portion of the drawing. Line 518 supplies
the acidic chemical to lines 435 and 475. Line 519 supplies
the alkaline chemical to lines 415 and 455. Line 520
supplies line 488, line 521 supplies line 468, line 522
supplies line 448, and line 523 supplies line 428. If
the chemicals in any of these stages are the same, then
only the single line need be used. The lines may also
represent multiple lines.
Although this has been described with water
as the washing fluid, the same flow patterns could be -
used with more expensive treating fluids such as alcohol
in order to preserve the alcohol within the system.
24
