Note: Descriptions are shown in the official language in which they were submitted.
~ 208 1 398
WASH PRESS MODIFICATION FOR
OXYGEN DErIG~IFICATION PROCESS
FIELD OF INVENTION
The present invention relates to a method for the
treatment of wood pulp, and more particularly to a
method for distributing alkaline material upon
brownstock pulp prior to oxygen delignification so
that a highly delignified pulp can be obtained after
oxygen delignification without deleteriously affecting
the strength of the pulp.
8ACKGROUND OF THE INVENTION
Wood is comprised in major proportion of
cellulose and hemicellulose fiber and amorphous, non-
fibrous lignin which serves to hold the fibrous
portions together. The hemicellulose and the
cellulose are sometimes referred to collectively as
holocellulose. During the treatment of wood to
20 produce pulp, the wood is transformed into a fibrous
mass by removing a substantial portion of the lignin
from the wood. Thus, processes for the production of
paper and paper products generally include a pulping
stage in which wood, usually in the form of wood
25 chips, is reduced to a fibrous mass. Several
different pulping methods are known in the art; they
are generally classified as mechanical, chemical or
semi-chemical pulping.
Chemical pulping methods include a wide variety
of processes, such as the sulfite process, the
bisulfite process, the soda process and the Kraft
process. The Kraft process is the predominant form of
chemical pulping.
Chemical pulping operations generally comprise
introducing wood chips into a digestinq vessel where
they are cooked in a chemical liquor. In the Kraft
process, the cooking liquor comprises a mixture of
~4 -
-- 2081 3~8
-- 2
sodium hydroxide and sodium sulfide. After the
required cooking period, softened and delignified ~ood
chips are separated from the cooking liquor to produce
a fibrous mass of pulp. The pulp produced by chemical
5 pulping is called "brownstock." The brownstock is
typically washed to remove cookinq liquor and then
processed for the production of unbleached grades of
paper products or, alternatively, bleached for the
production of high brightness paper products.
Since chromophoric groups on the lignin are
principally responsible for color in the pulp, most
methods for the bleaching of brownstock require
further delignification of the brownstock. For
example, the brownstock may be reacted with elemental
chlorine in an acidic medium or with hypochlorite in
an alkaline solution to effect this further
delignification. These steps are typically followed
by reactions with chlorine dioxide to produce a fully
bleached product. Oxygen delignification is a method
that has been used at an increasing rate in recent
years for the bleaching of pulp because it uses
inexpensive bleach chemicals and produces by-products
which can be burned in a recovery boiler reducing
environmental pollutants. Oxygen delignification is
frequently followed by bleach stages which use
chlorine or chlorine dioxide but require less bleach
chemical and produce less environmental pollutants
because of the bleaching achieved in the oxygen stage.
In some bleaching processes, the pulp is bleached
while being maintained at low to medium levels of pulp
consistency. Pulp consistency is a measure of the
percentage of solid fibrous material in pulp. Pulps
having a consistency of less than about 10% by weight
are said to be in the low to medium range of pulp
consistency. Processes which require bleaching at low
to medium consistency are described in the following
patents and publications: U.S. Patent 4,198,266,
_ - 3 - 2 0 8 1 3 9 ~
issued to Xirk et al; U.S. Patent 4,431,480, issued to
Markham et al; U.S. Patent Number 4,220,498, issued to
Prough; and an article by Kirk et al. entitled ~Low-
consistency Oxygen Delignification in a Pipeline
5 Reactor - A Pilot Study", TAPPI, May 1978. Each of
the foregoing describe an oxygen delignification step
that operates upon pulps in the low to medium
consistency range.
U.S. Patent 4,806,203, issued to Elton, discloses
~0 an alkaline extraction, preferably for chlorinated
pulp, wherein the timed removal of alkaline solution
is essential to prevent redepositing of dissolved
lignin back onto the pulp. If too short or too long
of a time period passes in this stage, the process
shows little benefit.
Oxygen delignification of wood pulp can be
carried out on fluffed, high consistency pulp in a
pressurized reactor. The consistency of the pulp is
typically maintained between about 20% and 30% by
20 weight during the oxygen delignification step.
Gaseous oxygen at pressures of from about 80 to about
100 psig is introduced into and reacted with the high
consistency pulp. See, G.A. Smook, Handbook for Pul~
and PaDer Technoloqists, Chapter 11.4 (1982). In
previous oxygen delignification operations, the pulp
after cooking is washed and dewatered to produce a
high consistency mat. The pulp mat is then covered
with a thin film or layer of an alkaline solution, by
spraying the solution onto the surface of the mat.
The amount of alkaline solution sprayed onto the mat
is about 0.8 to ~% by weight of oven dry pulp.
Previously used high consistency oxygen
delignification processes have several disadvantages.
In particular, it has now been found that spraying an
alkaline solution onto a mat of high consistency pulp
does not provide an even distribution of solution
throughout the fibrous mass, notwithstanding the
208 1 3q~
~_ - 4 -
generally porous nature of such mats. As a resul~ of
this uneven distribution, certain areas of the high
consistency mat, usually the outer portions, are
exposed to excessive amounts of the alkaline solutlon.
5 This excessive exposure is believed to cause
nonselective degradation of the holocellulosic
materials resulting in a relatively weak pulp, at
least locally. On the other hand, other portions of
the high consistency mat, typically the inner
10 portions, may not be sufficiently exposed to the
alkaline solution to achieve the desired degree of
delignification. Thus, overall quality declines.
SUMMARY OF THE INVENTION
The present invention provides a novel process
for obtaining enhanced delignification selectivity of
a pulp during a high consistency oxygen
delignification process wherein the oxygen delignified
pulp has greater strength and a lower lignin content
20 than has been attainable by prior art processes. In
addition, a wash press is utilized to apply a desired
quantity of alkaline material onto the pulp while also
reducing the amount of solids on the pulp which enters
the oxygen delignification reactor.
In accordance with the present invention, a first
amount of alkaline material is applied to an
unbleached pulp by washing the pulp in a wash press
with a solution of an alkaline material to a
consistency of at least about 18 percent by weight to
30 substantially uniformly distribute the first amount of
alkaline material throughout the pulp. This uniform
distribution of the first amount of alkaline material
is sufficient to assist in the enhancement of
delignification selectivity during high consistency
35 oxygen delignification compared to processes where the
alkaline material is applied upon high consistency
~_ - s - 2 0 8 1 3 9 ~
pulp or is applied at relatively low amounts onto low
consistency pulp.
In one embodiment of the invention, the pulp is
brownstock which is washed with a sufficient amount of
5 alkaline material so that the first amount applied to
the pulp equals the total amount. The pulp may be
mixed with dilution water to obtain a low consistency
pulp having a consistency of less than about 5 percent
by weight and preferably between about 1 and 4.5
percent by weight before being directed into the wash
press. The wash press increases the consistency of
the pulp to a high consistency, and removes pressate.
At least a portion of this pressate can be used as the
dilution water.
The pressate may contain alkaline material and,
if so, a portion may be used to wash the pulp prior to
directing the pulp to the wash press. Ideally, the
wash press is operated at a dilution factor at or near
zero and at a displacement ratio of as close to 1 as
possible to minimize the total alkali required. These
values are difficult to achieve and maintain in
commercial operation, however, so that the wash press
should be operated at a dilution factor of 0.4 or
les~, preferably less than 0.25 and at a displacement
ratio of at least about 0.5 and preferably above about
0.75.
In a second embodiment, a split alkaline material
addition sequence is used to apply the total amount of
alkaline material to the pulp. The first amount of
alkaline material is applied to the pulp in the wash
press while a second amount is applied to the high
consistency pulp. This reduces the amount of alkaline
material utilized in the wash press.
For either embodiment, a predetermined quantity
of pressate may be retained in a holding tank. This
quantity of pressate may be continuously returned or
recycled directly to the pulp/dilution water mixing
r
- 6 - 2 0 8 l 3 9 ~ -
step. The total amount of alkaline material applied to
the pulp is generally between about 0.8 and 7 percent by
weight based on oven dry pulp, and the pulp is then sub-
jected to oxygen delignification whereby enhanced delig-
nification is achieved.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a graph showing the relationship be-
tween pulp viscosity and K No. for softwood pulps
treated with alkaline material and delignified with
oxygen according to the present invention compared to
those representative of the prior art;
Figure 2 is a schematic representation of the pro-
cess of the present invention;
Figure 3 is a graph showing the relationship be-
tween percent viscosity change according to the propor-
tion of alkaline material applied to the pulp at high
consistency; and
Figure 4 is a graph showing the percent retention
of alkaline material on the pulp for operation of the
wash press at various displacement ratios.
DESCRIPTION OF PREFERRED EMBODIMENTS
The present invention relates to a process for
treating brownstock pulp with alkaline material prior
to high consistency oxygen delignification wherein the
pulp is substantially uniformly treated with the alka-
line material in a manner which minimizes usage of alk-
aline material and solids buildup on the pulp.
In U.S. Patent No. 5,085,734, it was demonstrated
that delignification selectivity across a high consis-
tency oxygen stage could be increased by substantially
uniformly distributing alkaline material throughout the
pulp. This distribution is achieved by mixing a solu-
tion of alkaline material with low consistency pulp
prior to pressing the pulp to high consistency. Thus, a
- ~ - 2 08 1 3 98
reduction in K No. in the pulp across the oxygen stage
of greater than 50% and generally at least 60% or more
can be achieved with essentially no damage to the
cellulose portion of the pulp.
The examples of the patent demonstrate the
unexpected deqree of improvement in delignification
selectivity compared to spraying the alkaline material
onto a pad of high consistency pulp after it emerges
from a press or other thickening device. This is
shown by the reduction in pulp K No. from about 10 to
26 upon entering the oxygen reactor (depending upon
the type of wood and the type of pulping conducted
upon the particular wood) to between about 5 and 10
after oxygen delignification with a change in
viscosity of the pulp from about 19 cps or more for
the incoming pulp to at least about 13 cps for the
oxygen delignified pulp. For softwood pulp, this data
is summarized and illustrated in the graph of Figure
1.
While this patented technique is successful, it
requires the use of greater amounts of alkaline
material compared to the spray applied process,
because much of the alkaline material is retained in
the pressate. The process of the present invention
achieves similar results in the uniform distribution
of alkaline material throughout the pulp by utilizing
a wash press to introduce the alkaline material onto
the pulp, while utilizing less alkaline material than
the patented alkaline material distribution process.
Furthermore, since the same amounts of alkaline
material are substantially uniformly distributed
throughout the pulp, the same advantages with regard
to increased delignification selectivity across the
subsequent oxygen delignification stage would be
achieved.
A wash press typically includes two synchronous,
counter-rotating, dewatering rolls configured and
~ 20~1398
designed to provide a dewaterin~ section, a wash
section and a final extraction section. Pulp enters
the press at a low consistency of about 3.5 percent by
weight in solution with dilution water. In the
initial dewatering section, the consistency is
increased to about l0 percent by weight and a pad is
formed on the rolls. In the wash section, wash water
is introduced onto the free side of the pad and begins
to be forced to pass therethrough, displacing the
10 dilution water in the pad. In the extraction (or
consolidation) section, additional dilution and wash
water is displaced from the pad to raise the
consistency of the pulp. The pulp then exits the
press at a high consistency of between about 25 and 3s
15 percent by weight.
To understand the present invention, one must be
familiar with the terms and conditions relating to
pulp washinq. The term "dilution water" is used to
refer to the water associated with the pulp as it
enters the press.
The term "wash water" is used to mean the water
that is added to the press independent of the pulp.
The term "dilution factor" is used to mean the
amount of wash water which is added to the wash press
in excess of that required to replace the amount of
water which exits with the pulp at the wash press
outlet consistency. For an amount of wash water which
is needed only to replace only that amount which
leaves with the pulp, the dilution factor ("DF") would
be zero. This factor is expressed as pounds of water
per pound of air dry ("AD") fiber (note: l pound AD
fiber equals 0.9 pound oven dry ("OD") fiber). The
dilution factor can be calculated from the following
formula:
~ 9 ~ 208 1 3 9 8
-
~ O . 9 1D OD fiDe~ x~ C )¦
where WT is the amount of wash water utilized (in
pounds per pound of air dry fiber; and
C is the consistency of the pulp exiting the
wash press.
For example, for a DF of zero and 28% consistency
pulp, the amount of wash water necessary to replace
that which leaves with the pulp is calculated as
follows:
[ ( 28 )]
or
WT = 2.31 pounds of water per pound of air dry fiber
An amount of water greater than this 2.31 value
20 provides an excess which leaves the wash press as
pressate.
The term "displacement ratio" is used to mean a
fraction representing the actual change in
concentration of a dissolved component in the dilution
25 water associated with the pulp exiting the wash press
divided by the maximum possible change, i.e., the
concentration of the component in the wash water less
the concentration of the same component in the
dilution water. The displacement ratio ("DR") can be
calculated from the following equation:
DR - (XO~ - Xw) / (Xw - Xw)
where Xa~ is the concentration of a particular
component in the dilution water leaving
with the pulp;
X~ is the concentration of the same
component in the dilution water
entering the wash press; and
208 1 39~
-- 10 --
Xw is the concentration of the same
component in the wash water.
It is readily observed that, when XO~ = Xw, the
displacement ratio is l, while for displacement ratio
5 of less than l,
XO~ = (DR)(Xw) + (l-DR)(X~)
For example, if DR = 0.75:
XO~ = 0.75(XW) + 0.25(X~)
while for a DR of 0.5:
XO~ = 0.5(XW) + 0.5(X~)
These washing concepts usually apply to the
removal of an unwanted component, such as black liquor
solids, from the pulp where the concentration of the -
component in the wash water is substantially less than
in the dilution water. In the present invention,
however, these relationships are utilized to
illustrate how the concentration of a component in the
wash (such as alkaline material) water can be made
higher than its concentration in the dilution water so
20 that the component can be added to, rather than
removed from, the pulp.
As is evident from the formulae presented above,
with a dilution factor of zero and a displacement
ratio of l, all alkaline material added to the wash
25 water (at a concentration of Xw) will theoretically be
retained upon the pulp as it exits the wash press.
While it is not possible to commercially operate the
wash press to achieve complete displacement and
substitution of the wash water for the dilution water
30 cont~ined in t~e pulp, the process should be operated
at dilution factors of below about 0.4 and at
displacement ratios above 0.5. This provides three
advantages for the process of the present invention.
l) very little alkaline material is lost from
the wash press into the pressate;
- 11 - 208 1 3 98
._
2) rapid responses to pulp chan~es can be
achieved by changing the concentration of
alkaline material in the wash water (Xw);
and
3) sufficient washing of the pulp occurs in the
press so that the carryover of dissolved
black liquor or other contaminants into the
oxygen reactor is siqnificantly reduced.
Another advantage of this embodiment of the present
1O process is that the control strategy and equipment for
the introduction of alkaline material-is simplified,
in that only one addition point is required.
The present invention therefore provides high
quality, high strength, delignified wood pulp from
15 Kraft pulp or pulps produced by other chemical pulping
processes. The preferred starting material is
unbleached pulp obtained by cooking wood chips or
other fibrous materials in a cooking liquor, such as
by the Kraft or Kraft AQ process.
With reference to Figure 2, wood chips 1 and a
white liquor 2 comprising sodium hydroxide and sodium
sulfide are introduced into a digester 3. Sufficient
white liquor should be introduced into the digester to
substantially cover the wood chips. The contents of
25 the digester are then heated at a temperature and for
a time sufficient to allow the white liquor to
substantially impregnate the wood chips and
substantially complete the cooking reaction.
This wood chip cooking step is conventionally
30 known as Kraft cooking or the Kraft process and the
pu7p produced by this process is known as Kraft pulp
or Kraft brownstock. Depending upon the
lignocellulosic starting material, the delignification
results obtained with the conventional Kraft process
35 may be increased by the use of extended
delignification techniques or the Kraft-AQ process.
Moreover, these techniques are preferred for obtaining
- 12 - 208139~
the greatest degree of reduction in ~ No. of the pulp
without deleteriously affecting the strength and
viscosity properties of the pulp during the cooking
stage.
When using the Kraft-AQ technique, the amount of
anthraquinone in the cooking liquor should be an
amount of at least about 0.01~ by weight, based on the
oven dried weight of the wood to be pulped, with
amounts of from 0.02 to about 0.1% generally being
tO preferred. The inclusion of anthraquinone in the
Kraft pulping process contributes significantly to the
removal of the lignin without adversely affecting the
desired strength characteristics of the remaining
cellulose. Also, the additional cost for the
15 anthraquinone is partially offset by the savings in
cost of chemicals utilized in the bleaching steps
which follow oxygen delignification of the pulp.
Alternatively or additively to Kraft-AQ is the
use of techniques for extended delignification such as
the Kamyr MCC, Beloit RDH and Sunds Super Batch
Methods. These techniques also offer the ability to
remove more of the lignin during cooking without
adversely affecting the desired strength
characteristics of the remaining cellulose.
The digester 3 produces a black liquor containing
the reaction products of lignin solubilization
together with brownstock 4. The cooking step is
typically followed by washing to remove most of the
dissolved organics and cooking chemicals for recycle
and recovery, as well as a screening stage in which
the pu~ p is passed through a screening apparatus to
remove bundles of fibers that have not been separated
in pulping. The brownstock 4 is then directed to a
blow tank 5.
Brownstock 6 exiting blow tank 5 ls washed in
washer 7 generally with effluent 8 generated from the
washing of pulp downstream of the oxygen
~r
208 1 39~
- 13 -
delignification reactor. The washed pulp g exits the
~asher ~ at a consistency of about 15 to 18 percent by
weight and is directed to a dilution tank 10, where it
is diluted to a consistency of about 3.5 percent by
5 weight. The dilution water on startup is supplied by
filling the tank 10 with water 11. The pulp/dilution
water stream 12 is then directed into wash press 13
where alkaline material is added to the pulp.
A preferred wash press 13 is Displacement Press
0 DP, which is available from Sunds Defibrator AB,
Sundsvall, Sweden. Other twin-roll presses may also
be used, provided that they possess the combination of
dewatering, washing and displacement of the pulp, as
described above. The Sunds device includes two
15 synchronous, counter-rotating dewatering rolls in a
pressurized vat that has dual pulp inlets. A pad of
pulp having a consistency of about 10 percent by
weight is formed on the rolls at the end of the
dewatering section.
A solution 14 of sodium hydroxide, oxidized white
liquor, or other equivalent alkaline material is used
to wash the pulp in the wash press 13. This
solution 14 is preferably prepared by mixing
concentrated alkaline material with bleached pulp wash
25 water effluent. Then, solution 14 is introduced on
the free side of the pulp pad and begins to move
through the pulp pad to displace the dilution water
therein. By attempting to operate at a displacement
ratio of close to 1 and a dilution factor of close to
zero, most of the alkaline material used to wash the
pulp is substantially uniformly distributed throughout
the pulp and is retained thereon. As noted above, the
wash press 13 should be operated at a dilution factor
of less than 0.4 and a displacement ratio of at least
35 about 0.7 to achieve the desired distribution of
alkaline material on the pulp. The alkaline material
containing pulp then exits the wash press 13 at a
~r
- 14 - 2 0 8l 3q 8
consistency of 25 to 35 percent by weight. In a most
preferred embodiment, the pulp 15 has a consistency of
about 28 to 29 percent by weight.
The wash press 13 enables reduced quantities of
5 alkaline material to be used for application of the
deslred amount onto the pulp compared to e.g., the use
of a chest to add to alkaline material to the pulp.
Also, the amounts of organic and/or inorganic solids
on the pulp exiting the wash press 13 are reduced, so
l0 that less of these contaminants are carried by the
pulp into the oxygen reactor. Thus, lesser amounts of
chemical are consumed during the oxygen reaction with
the pulp. In addition, compared to a conventional
washer, less alkaline material is lost to the plant
liquid recovery system due to pressate discharge or
due to "breakthrough" of pressate into the washer
effluent.
One skilled in the art can select the appropriate
quantities ti.e.~ concentrations and flow rates) of
20 alkaline solution 14 to be added to the wash press in
this step to achieve a distribution of the desired
amount of alkaline material throughout the pulp. In
particular, the aqueous sodium hydroxide solution 14
is combined with the pulp in an amount sufficient to
25 provide at least about 0.8 to about 7 percent by
weight of sodium hydroxide on pulp based on OD pulp
after the consistency is increased by the wash press.
As noted above, other sources of alkaline material
having equivalent sodium hydroxide content can also be
employed, such as oxidized white liquor.
The wash press 13 also removes residual
liquid 16, called pressate or wash filtrate, from the
pulp. Also, alkaline material which is not retained
on the pulp due to less than ideal displacement will
be included in pressate 16. This pressate 16 is
directed to a holding or seal tank 17, which can be
used to achieve smooth, continuous operation of the
~, .
- 15 - 208 1 3 ~
process by retaining all pressate 16 from wash press
13. At least a portion 18 of pressate 16 is recycled
to the dilution tank 10. Upon startup, holding tank
17 is filled with a suitable source of dilution water
5 11, generally fresh water. During continuous
operation of the process, pressate portion 18 is
returned to the dilution tank 10 and constitutes
substantially all the dilution water. Thus, stream 11
is utilized only upon startup (i.e., before pressate
is generated by the wash press 13) or when additional
dilution water is needed. Alternatively, a portion 19
of the pressate can be used to wash the pulp upstream
of wash press 13 at washer 7 as part of stream 8.
Also, as shown, a split shower can be used on
15 washer 7, with part of the wash water provided by
stream 8 and part by stream 19. Pressate portion 19
can be discharged to the plant recovery system as
necessary to maintain water balance in the dilution
tank/wash press circulation loop.
For the situation where significant amounts of
alkaline material are introduced into the pressate due
to imperfect displacement of the wash water 14 into
the pulp, the concentration of the alkaline material
in the wash water 14 can be increased to compensate
for such imperfect displacement. The recycling of the
pressate to the dilution tank raises the concentration
of alkaline material in the dilution water, thus
enabling the pulp/dilution water stream 12 to have a
greater content of alkaline material. This recycling
increases alkaline material retention in the pulp to
the desired level at the expense of a slight lowering
in the degree of washing action and the reduction of
carryover of undesired material on the pulp 15 which
exits the wash press.
One skilled in the art having the benefit of this
disclosure before them can determine by routine
experimentation the appropriate alkaline material
208 1 39~
- 16 -
concentrations for the wash water 14 based on the actual
dilution factors and displacement ratios used to operate
the wash press 13 to apply the desired amount to the
pulp .
In a second embodiment of the invention, which is
also shown in Figure 2, a first portion of the total
amount of alkaline material necessary for application to
the pulp for the high consistency delignification in the
oxygen reactor 21 is added to the wash press 13 in the
manner described above.
In U.S. Patent No. 5,173,153 it was demonstrated
that the application of about half of the desired amount
of alkaline material to the pulp in a substantially uni-
form distribution, such as by mixing low consistency
pulp with a solution of an alkaline material, followed
by applying the remaining half of the amount by spraying
onto high consistency pulp enables the pulp to achieve
comparable increased delignification selectivities com-
pared to the application of all alkaline material to the
low consistency pulp. In addition, the quantity of alk-
aline material needed to apply the desired amount onto
the pulp was reduced compared to the full low consis-
tency application, while the spraying of a portion of
the alkaline material onto the high consistency pulp
enables that process to provide rapid modification or
adjustment of the total amount to be applied to the pulp
to compensate for changes in the properties of the pulp
or to vary the degree or extent of the oxygen delignifi-
cation of that pulp.
Examples 6 to 9 of that patent application illus-
trate the benefits obtainable by application of the al-
kaline material according to the split addition process.
Figure 3 of the present invention illustrates the effect
of increasing the percentage of
h
~ - 17 - 208139~
proportion of alkaline material utllized in treating
the high consistency pulp. The solid horizontal line
proximate to the 0 viscosity change numeral
corresponds to the baseline viscosity achieved with
5 all alkaline material applied to low consistency pulp.
The two broken horizontal lines on either side of the
solid 0 line delineate the boundaries of a typical +
6% deviation in viscosity. As is evident from Figure
3, as the amount of alkaline material applied onto the
0 high consistency pulp exceeds about 50~ of the total
applied to the pulp, the viscosity of the delignified
pulp drops below the acceptable deviation. The
benefits of enhanced delignification selectivity are
also reduced for applications of 50% or more of the
alkaline material to the high consistency pulp.
Therefore, at least about 50% and preferably from
about 55 to 90% of the alkaline material should be
applied to the low consistency pulp, with the balance
applied to the high consistency pulp.
The present invention contemplates similar
advantages by applying about half of the total amount
of alkaline material during the wash press 13
treatment described above. Thereafter, a second
amount of additional alkaline material is applied to
ths high consistency brownstock 15 exiting the wash
press 13 by conventional spray techniques to obtain
the desired total amount of alkaline material on the
pulp prior to oxygen delignification. This second
amount of alkaline material is selected to apply the
remaining amount necessary (again, about one-half of
the total amount) to achieve the desired extent of
delignification in the subsequent oxygen
delignification step which is carried out on the
alkaline material treated high consistency pulp.
As noted above, the total amount of alkaline
material actually applied onto the pulp will generally
be between 0.8 and 7 percent by weight based on OD
- 18 - 20 8 1 3 q ~
pulp, and preferably between about 1.5 and 4 percent
by weight for southern softwood and between about 1
and 3.8 percent by weight for hardwood. In the
alternate embodiment of Figure 2, about half these
5 amounts are preferably applied in each of the low
consistency and high consistency treatments. Thus,
about 0.5 to 2 percent by weight, preferably about 0.5
to 1.9 percent by weight for hardwood and 0.75 to 2
percent by weight for softwood, is applied onto the
10 pulp during each of the low and high consistency pulp
alkaline treatments.
The split addition process of this invention
further improves control of the addition of alkaline
material to the pulp. The high consistency alkaline
15 treatment step allows rapid modification or adjustment
of the total amount of the alkaline material present
in or upon the pulp which will enter the oxygen
delignification reactor 21. The combination of easy
control of both the amount of alkaline material added
20 to the wash water of the press with the amount sprayed
onto the high consistency pulp creates an optimum
response to changing delignification requirements in
that the precise total amount to be applied to the
pulp can be eaaily and rapidly varied to compensate
for changes in the properties (i.e., wood type, K No.
and viscosity) of the incoming brownstock, or to vary
the degree or extent of oxygen delignification for a
particular pulp.
The fully alkaline material treated pulp 15 is
3~ then farwarded to the oxygen delignification reactor
21 where it is contacted with gaseous oxygen 22 by any
of a number of well known methods. Suitable
conditions for oxygen delignification according to the
present invention comprise introducing gaseous oxygen
35 at about 80 to about 100 psig to the high consistency
pulp while maintaining the temperature of the pulp
between about 90 and 130C. The average contact time
~ - 19 - 208139~
between the high consistency pulp and the gaseous
oxyqen ranges from about 15 minutes to about 60
minutes.
After oxygen delignification in reactor 21, the
5 delignified pulp 23 is washed to remove dissolved
organics and to produce a high quality, low color
pulp. As noted above, the effluent from this washing
stage can be recycled for use on washer 7 or for
introduction as stream 11 for dilution tank 10. A
portion of this stream is preferably utilized as
stream 14 after the addition of concentrated sodium
hydroxide or oxidized white liquor thereto. The
oxygen delignified pulp can be used to form paper
products of unbleached brownstock, or can ~e sent to
subsequent bleaching stages to produce a fully
bleached product.
Additional advantages of the present invention
can be obtained during the subsequent bleaching of the
oxygen delignified pulp 23. Such bleaching can be
conducted with any of a wide variety of bleaching
agents, including ozone, peroxide, chlorine, chlorine
dioxide, hypochlorite or the like. When conventional
chlorine/chlorine dioxide bleaching processes are used
to increase the degree of brightness of the pulps
which have been treated with alkaline material as
de~cribed above, a substantially reduced amount of
total active chlorine is used compared to the
bleaching of pulps which are oxygen delignified by
prior art techniques. The total amount of chlorine-
containing chemicals utilized according to the presentinvention is reduced compared to the amount needed for
the same starting pulp which is not treated with
alkaline material in a wash press. Similarly, when
the chlorine/chlorine dioxide treated pulp is followed
by an alkaline extraction stage, substantially reduced
amounts of alkaline material are needed in this stage
compared to a bleaching process for pulps which have
- 20 - 208139~
not been uniformly combined with alkaline material at
low consistency. The amount of alkallne material
utilized in the extraction step would also be reduced
for pulp treated with alkaline material at in a wash
5 press as disclosed herein.
In addition to providing cost advantages with
respect to the reduced amounts of chemical necessary
for such treatments, the process of the present
invention also reduces the amounts of environmental
pollutants caused by the use of chlorine, since
reduced amounts of chlorine are used. Furthermore,
due to the lower usage of chemicals in this portion of
the system, the amount of contaminants in the waste
water from the plant which is to be treated is
l5 correspondingly reduced with similar savings in waste
water treatment facilities and related costs.
EXAMPLES
Example 1:
A mathematical model was used to illustrate the
process of the present invention on softwood pulp as
described in Figure 2 with the wash press operated at
a dilution factor of 0 and a displacement ratio of
O.9S. For this embodiment, pressate portion 19 is
utilized as wash water 8 on washer 7. The flow rates
for the applicable streams would be as follows:
Process Flow Rate Pulp Sodium
Stream liquid pulp Consistency Hydroxide
(Fiq. 2~ (qDm~ (t/d) (%) (lb/hr)
g s04.2 s80.8 18 209.5
12 2726.5 580.8 3.5 1533.8
14 247.9 ~ 1213.3
312.2 580.8 28 1159.8
16 2662.2 -- -- 1587.3
19 440.0 -- -- 262.0
Thus, the table shows that the pulp exiting the wash
press (stream ~5) at a flow rate of 580.8 t/d has a
''X
2d~1398
- 21 -
consistency of 28% and contains 1159.8 lb/hr of sodium
hydroxide. This calculates to an amount of sodium
hydroxide applied to the pulp of about 2.4%. If the
wash press could be operated at a displacement ratio
5 of 1, then all alkali added to the wash press by
stream 14 would be applied to the pulp leaving the
press at stream 15. For operation at displacement
ratios of less than 1, a portion of the alkaline
material added to the wash press by stream 14 will be
lost into the pressate.
ExamDle 2:
To illustrate the transfer of alkaline material
to the pulp when operating at different displacement
ratios, the graph of Figure 4 is presented. This
illustrates a mathematical model for a wash press that
is operated at a dilution factor of 0 and shows how
the amount of alkaline material applied to the pulp
can vary as the displacement ratio is changed. In
20 this model, the pressate is recycled to both the
dilution tank and stream 18 and to brownstock washer 7
as a portion of stream 8. The results thus show the
benefit of increased alkaline material retention on
the pulp due to recycling.
While it is apparent that the invention herein
disclosed is well calculated to fulfill the objects
above stated, it will be appreciated that numerous
modifications and embodiments may be devised by those
30 skilled in the art, and it is intended that the
appended claims cover all such modifications and
embodiments as fall within the true spirit and scope
of the present invention.
