Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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C~LORINE FREE PULPING AND BL~ACH NG S_QUENC~
B~CKGROUN~ AND SU~ffl~RY OF T~E INVENTION
In the production of bleached kraft pulp there
have been increasing reyulatory and market demands
to reduce or eliminate the presence of chlorinated
organic compounds in pulp products and bleaching
effluents. In order to properly react to such
demands, it is necessary to eliminate the use of
chlorine gas or any other chlorine containing
compound (e. g. chlorine dioxide). However the
removal of the chlorine based bleaching agents makes
it extremely difficult to achieve desired brightnes~
levels, especially if the pulp produced is to have
acceptable strength properties. For example oxygen
delignification must be utilized, and multiple stage
oxygen delignification -- especially w.ith chelating
treatment to control deleterious metal ions -- has
been shown to offer advantages in delignification
and selectivity, especially when there is between-
stage washing (see U.S. patent 4,946,556). However
there are practical limits governing both the exten1-
of delignification and the qual.ity of pulp
obtainable using oxygen delignification alone.
Of other available bleaching agents, perhaps
the most promising is ozone. It has been
demonstra-ted th~t ozone is a highly effective
delignifying agent, however ozone has seen only
marginal commercial acceptance to date in the pulp
industry. The main obstacles to commerci.al
utilization of ozone have been the chemical cost of
ozone when large doses are needed, and the
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degradation of pulp strength that typically occurs
relative to conventionally bleached pulps.
Thus, there has long been a need for bleached
kraft pulp with commercially acceptable strength
properties without the use of ch]orine based
bleaching agents. According to the present
invention, and that of the parent applicati.on, i-t is
possible to finally achieve that goal.
According to the present invention, the basic
approach that is taken is to have a pulp with
minimum Kappa Number and maximum strength before it
is subjected to ozone, and subsequent non-chlorine,
bleaching sequences. ~his is accomplished according
to the present in~ention by utilizing kraft pulp
produced from extended delignification processes.
Continuous digesters sold by Kamyr, Inc. of Glens
Falls, New York under the trademark "EMCC" practice
a process known as extended modified continuous
cooking. Such a process can produce softwood pulps
having a very low Kappa Number, typically about
18-20, while maintaining a high pulp viscosity --
comparable to that for a conventional kraft (CK)
pulp at a Kappa Number of 30. In the practice of
the method according to the invention, a hot alkali
extraction is utilized to remove lignin from the
pulp after treatment in an "~MCC" digester, metals
are typically removed, and then the pulp is
subjected -to ozone bleaching stages.
~ ccording to one aspect of the present
invention, there is provided a method of
continuously kraft pulping and then bleaching
; comminuted cellulosic fibrous materia1 using an
upright digester having top, bottom, and cen-tral
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portions, comprising the following steps: ~a)
Passing comminuted cellulosic fibrous material
entrained in kraft white liquor into the top of the
digester. (b) Extracting black liquor from at least
one screen between the top and bottom of the
digester. (c) At a first portion of the digester
withdrawing and recirculating liquid in a first
recirculation loop. (d) ~dding kraft white liquor
to the first recirculation loop. (e) Adjacent the
bottom of the digester withdrawing and recircula-ting
liquid in a second, wash, recirculation loop. (f)
Adding kraft white liquor to the second
recirculation loop, the liquor recirculated into the
digester in part passing upwardl~ therein
countercurrent to the material flow, the amount of
white liquor added in the second recirculation loop
being sufficient to increase the viscosity and
strength properties of the pulp produced compared to
the practice of the same method with the same
material, Kappa Number, and other parameters only
without step (f). (g) Withdrawing kraft p~llp from
the bottom of the digester, steps (a?-(f) being
practiced to produce pulp having a Kappa Number
comparable to about 20 or below for softwood. (h)
Subjecting the pulp withdrawn from the bottom of the
d.igester to a hot alkali extraction to extract
lignin therefrom without degradation of the pulp
fibers; and then (i) effecting bleaching of the pulp
with non-chlorine containing bleaching chemicals to
obtain a bleached pulp having propertie~ comparable
to that obtained by treating CK pulp with a DEDED
sequence.
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The bleaching chemicals utilized in the
practice of step (i) are, first, oxygen (0), and
peroxide (P) and ozone (Z); a typical bleaching
sequence is OOAZEZP, with or without a following Z
stage. The A (acid wash) stage effects removal of
heavy metals, which are purged from th~ system when
some of the wash filtrate is diverted. The
extraction (E) stage may be an oxidative peroxide
extraction stage, or a conven~ional oxidative
extraction stage. The o~one bleaching stages are
practiced with the pulp at a consistency of about
5-18%.
The method is typically practiced utilizing an
impregnation vessel, and a conduit connected between
the impregnation vessel and the diyester; and step
(a) is practiced by adding kraft white liquor to the
conduit and impregnation vessel, and so that the
majority of the kraft white liquor added is added in
the conduit and the impregnation vessel, about 5~20%
of the kraft white liquor utilized to effect kraft
cooking is added in step (f), and about 10-20% of
the kraft white liquor util.ized is added in step (d~
According to another aspect of the present
invention, there is provided a method of producing
bleached kraft pulp, comprising the s-teps of
se~uentially: (a) Subjecting kraft pulp to hot
alkali extraction, to remove lignin from the pulp
without degradation thereof. (b) Subjecting the
pulp to oxygen bleaching. (c) ~emoving a-t leas~
some heavy metals from the pulp; and (d) ozone
bleaching the pulp, whi.le it has a consistency of
about 5-1~%. Step (c) is practiced by acid washing
the pulp to produce a wash filtrate, a-d removing
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part of the filtrate to prevent the bu:ildup of heavy
metals. Step (d) is practiced by the bleaching
sequence ZEZP or (ZE)P(ZE)P, with or without a fina]
Z stage.
It is the primary object of the present
invention to provide for the production of kraf-t
pulp having acceptable brightnass (e.g. greater than
90 CPPA) and commercially acceptable strength,
without utilizing chlorine containing bleaching
compounds. This and other objects of the invention
will become clear from an inspection of the detailed
description of the invention, and from the appended
claims.
BRIEF DESC~IPTION OF T~E DRAWINGS
FIGUREs lA and lB provide a schematic view
showing apparatus for practicing the exemplary
method according to the invention in which an
extended modified continuous cooking process is
employed to produce pulp, followed by hot alkali
extraction and non-chlorine bleaching sequences; ancl
FIGURE 2 is a schemati.c view of the wash
aspects of the treatment sec~uence of FIGUREs lA & lB
showing the lic~uid flow interconnections between the
various stages.
_TA LE~_DE_CRIPTION OF ~HE_DR~WING5
An exemplary apparatus for kraft pulping
according to the .inventi.on is illustrated in FIGURE
lA. Rntirely standard components include a chips
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bin 10 for receiving a ~eed of chips or like
cellulosi~ comminuted fibrous material. From the
chips bin 10, the material ~oes to a horizon-tal
steaming vessel 12, and a vertical conduit 13, kraft
white liquor from the source 14 bein~ added to the
material in the conduit 13 to slurry the material as
it is fed to the high pressure feeder 15. From the
high pressure fe~der 15 the material entrained in
kraft white liquor passes to an optional
impregnation vessel 16, and then in line 17 to the
top 18 of a continuous digester 19. At the top of
the digester 19 there is a liquid/material
separation device 20, which preferably comprises a
plurality of bull screens with switch.ing withdrawal
from the screens, or the like. The digester 19 also
includes a central portion 21 and a bottom 22.
At a portion of the digester 19 between the top
18 and the central portion 21 thereof one or more
screens 23, and conduit 24, are provided a.s means
for extracting black liquor from the digester 19.
The black liquor is typically passed in conduit 24
to a series of flash tanks, as is conventional. At
the central portion 21 of the digester 19,
withdrawal screens 27 are also provided, bei.ng
connected by a withdrawal conduit to a pump 28 and a
heater 29 for re-introduction of the withdrawn
liquid at point 30 of a re-introduc-tion conduit, the
point 30 typically being slightly above the screens
27. At the bottom 22 of the digester, kraft pulp is
withdrawn in conduit 33 (a scraper or like
conventional components can also be u-ti.lized), and
wash liquor from source 34 is added in introduction
conduit 35.
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Adjacent the bottom of the digester ]9 a wash
screen (one or more rows) 37 is provided, liquid
being withdrawn therethrough in a withdraw~l conduit
under the influence of pump 38, and then passing
through heater 39 into a re-introduction conduit to
be introduced at point 40 just above the screen 37.
From the point 40 up to the screens 23, liquid flows
countercurrently to the material -- as designa-ted by
arrows 42 -- while above the screens 23 it flows
cocurrently.
If desired, white liquor having higher
sulfidity ~ay be supplied to the vessel 16 than to
the digester 19; or at other desired points in the
system, the wood chips may first be treated with
high sulfidlty liquor, and then normal white
li~uor. That is, white liquor added prior tG the
beginning of the cooking reaction may have e~lanced
sulfidity relative to the average sul:fidity of the
mill's white liquor supply (14), and also pref~rably
has an elevated level of sodium polysulfide.
What has been described so far, and including a
main conduit 44 for adding kraft white liquor i.n
conduit 45 to the first recirculation loop at
digester central portion 21, is conventional i.n the
Kamyr MCCIM system. In accordance with the present
inVention, a conduit 46 is provided for
interconnecting the mai.n conduit 44 to the
~ithdrawal conduit in the second wash recirculation
loop provided by elements 37 through 40.
In the practice of the present invention,
sufficient kraft white liquor, and additional heat,
are added in conduit 46 so as to achieve
significantly enhanced viscosity and strength
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properties of the pulp produced compared to the
practice of the same method without -the introduction
of kraft white licluor in conduit 46. For example,
the amount of kraft white lic~or added in conduit 46
is at least about 5% of the total amount of kraft
licluor utilized -to effect kraft pulpin~, and
typically is about 10-20% (e.g. about 15%). Kraft
white licluor preferably is also added -- as is known
per se -- in conduit 45 in addition to conduit 46,
the amount added in conduit 45 being at least about
10%, and preferably about 10-20%.
The apparatus of FIGURE lA also includes a
recirculation loop (not shown) at a central portion
-- indicated generally by reference numeral 49 --of
the impregnation vessel 16, including a screen,
withdrawal conduit, pump, and re-introduction
conduit/point. Kraf-t white liquor from source 14
also is preferably added as indicated by lines or
points 52, 53, 54 to the recirculation loop in the
impregnation vessel 16, and to the recirculation
conduits from the digester to the impregnation
vessel 16, and from the impregnation vessel 16 to
the high pressure feeder 15. Normally the majority
of the white liquor used in the conventional
continuous kraft pulping process is added at the
points or conduit5 52 - 54. The extended
delignification pulp in line 33 is then subjected to
an optional further wash stage 58, and other
treatments.
Ater optional wash stage 58, the pulp passes
to a hot alkali extraction vessel/stage 60. Hot
alkali extraction (HAE) is shown per se in Einnish
patent docllment 865135/1986 ancl i.ncludes treatmen-t
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of the pulp With a mild alkali solution at a
temperature of about 150 degrees C for a time
sufficient to extract residual lignin from the pulp
(typically about thirty minutes or less). The pulp
may pass from HAE stage 60 to a pressure diffuser 61
or the like (e. g. a DDTM washer sold by Ahlstrom
Recovery of Roswell, Georgia) for washing. In -the
diffuser 61, white liquor may be added to enhance
d~lignification, as shown in U. S. Patent
5,021,127. The pulp may then pass to oxygen s-tages
62, 63, which may be of the type shown per se in
U.S. patent 4,946,556. At this point, the Kappa
number of the pulp has been reduced between about 10
and 5 units compared to CK softwood pulp, and
between about 5 and 2 units compared to CK hardwood
pulp.
After the optional second (or subsequent)
oxygen stage 63, the pUlp passes to a pressure
relief vessel 64, and then to diffusion washer 65 or
the like on top o vessel 66. Acid and enz~nes may
be added to the pulp in the tank 66 to facilitate
removal of heavy metals in a subsequent acid wash
stage. Separation of undesirable part.icles and
screening take place at 67, 68, typically at a
consistency of about 3-4 % and under pressure.
Alternatively, the screening system (like structures
67, 68) may be located in front of the oxygen
vessels 62, 63 in the process sequence.
In the acid wash stage 69 (FIGURE lB), some of
the wash filtrate is removed and passes to cooking
li~uor prepara-tion 70. This removed filtrate
contains heavy metals, and is accomplished to
prevent buildup of heavy metals in the pulp. The
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pulp then is ready to be bleached with non-chlorine
(e.g. chlorine and chlorine dioxide free) bleaching
chemicals, yet it will produce pulp comparable to CK
pulp produced by a DEDE~ sequence.
Although chlorine free treatment is vastly
preferred, according to the invention better quality
pulp can also be produced by using the EMCCIM
treatment followed by hot alkali extraction.
The first bleaching staye is preferably an
ozone (Z) stage, 71, which is practiced with the
pulp at medium consistency (i.e. about 5-18%). In
the Z stage, the ozone in an oxygen carrier gas
being introduced under super atmospheric pressure,
typically with about 2-14% of the introduced gas
comprising ozone.
After ozone stage 71, the pulp passes to tanks
72 or the like, is washed as indicated at 73, and
then subjected to an extraction stage (E) in vessels
74 and/or 75. AlternativeJy, -there can be no
washing between 71, 74, i.e. a (ZE) stage instead of
ZE stages. The ~essels 74, 75 typically provide an
oxidative pero~ide extraction stage (sometimes
designated Eop). Ater the extraction stage wash
se~uence at 76, bleaching preferabl~ contlnues in Z
stage 77 (comparable to stage 71), which is
connected to tank 78 which maintains the pulp in
contact with added peroxide, and after the ozone
stage 77 and washing in washer 79, the pulp pa~ses
to peroxide (P) stage 80
; After P stage 80 washing in washer 81, the pulp
may pass to ano-ther, optional, Z stage 82 (the same
as stages 71, 77, with medium consistency
treatment), and from storage tanks ~3 pulp may be
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withdrawn in the bleached pulp discharge line ~4.
The pulp in line 84 has properties comparable to
conventional (CK) pulp subjected to a DEDED
sequence. The bleaching chemical consumption is
also comparable to a DED~D sequence. The following
Table I provides an analysis of expected chemical
consumption from the OOAZEZPZ sequence described
above, while Table II indicates the expected Kappa
number and yield loss associated with each stage of
the sequence.
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TABLE I
Ozone bleaching - chemical consumption
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Stage O A ZEOp Zp _ Z
Oxygen kg/adt 15 5 2 . .
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Ozone kgladt 4 2 2
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Peroxide kg/adt 15 5 ~
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Ox. White liquor kg/adt 12
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Alkali kg/adt 18 6 6
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H2SO4 kg/adt 7 3 1
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MgSO4 kg/ adt 3 1 1 1
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Na2EDTA kg/adt 1. 5 0 . 5 O . 5 O . 5
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TABLE I I
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Kappa number ¦ Yield loss
l E~CCTM 20.0 0.7%
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¦ HAE 15.0 1.3%
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0.0 8.0 O.lg~
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P~ ~.0 0.1%
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Z 4.0 0.6%
_ ~ _ - 1. 3 %
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ZPO <1 . 0 0 . 25%
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P ~l.o 0.25%
Z <1 . ~) <O . 1%
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Process collditions
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The oxygen (with small amounts of ozone) gas
from the various stages 71, 77, 82 is captured in an
oxygen recovery system 84'. After scrubbing to
remove fiber, and after ozone destruction, some of
the recycled oxygen may be passed to oxygen
bleaching (e.g. stages 62, 63) via line 85', while
the rest, in line 86', passes to the atmosphere or
other uses within the p~lp mill, or is recycled to
the ozone production system.
E'IGURE 2 illustrates details of the washing
sequences associated with and/or between the various
stages described with respect to FIGURRS lA and lB.
The pulp in line 85 is from oxygen deliynification,
and is further washed in diffusion washer 86, with
liguid return via line 87 to the brown stock
washer. The washed pulp from diffusion washer 86
passes to acid washer 88 (comparable to 69 in FIGURE
lB), and is supplied with fresh water at 89. The
filtrate in line 90, which comprises a variable, bu1-
significant, amount of the total filtrate rom
washer 88, is sent to cooking liquor preparation
(e.g. 70 in FIGURE lB), to prevent buildup of heavy
metals.
From washer 88, the pulp ultimately flows to
washer 91 (comparable to 73 in FIGURE lB~ after the
first ozone stage, with wash liquid from that washer
being fed by line 92 to the washer 88 to provide
part -- with the fresh water added at 89 -- of the
wash liquid thereto. The pulp then ultimately flows
to the washer 93 after the extraction stage, with
recycle via line 94 back to the washer 91, and as
the wash liquid for diffusion washer 86. Fresh
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water also provides part of the wash water to washer
93 (comparable to 76 in FIGURE lB).
From washer 93 the pulp ultimately passes to
washer 95 (comparable to 79 in FIGURE 1~) after the
next Z stage, with recycle via line 96, and then to
washer 97 (comparable to 81 in FIGURE 1~) after the
P stage, with recycle via line 98. Fresh water is
added at 99 to the washer 97 as the wash liquid.
The pulp discharged in line 100 may pass to a final
Z stage, and then becomes the final pulp (in line 84
from FIGURE lB).
The washers 69, 73, 76, 79, ~1, 88, 91, 93, 95,
and 97 may be drums, diffusers, belts, presses, or
the like, as long as they provide adequate washing
efficiency. The washer 61 must be a pressurized
washer.
A portion of the washing filtrate from lines
92, 94, 96, 98, 101, 102, and/or 103 may be purged
from the system to the acid or caustic sewer in
order to maintain acceptable dissolved solids and
metals concentration levels in the system.
It will thus be seen that according to the
present invention a high brightness, high strength
bleached kraft pulp can be produced without chlorine
based bleaching compounds. While the invention has
been herein shown and described in what i.s presently
conceived to be the most practical and preerred
embodiment it will be apparent to those of ordinary
skill in the art that many modifications may be made
thereof within the scope of the invention, which
scope is to be accorded the broadest interpretation
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of the appended claims so as to encompass all
eq~ivalent processes and methods.
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