Note: Descriptions are shown in the official language in which they were submitted.
2069447
PULP ALKALI ADDITION PROCESS
FOR HIGH CONSISTENCY OXYGEN DELIGNIFICATION
FIELD OF INVENTION
The present invention relates to a method for the
treatment of wood pulp, and more particularly to a method
for oxygen delignification of brownstock to produce highly
delignified pulp without deleteriously affecting strength.
BACKGROUND 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 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 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 digesting vessel where they
are cooked in a chemical liquor. In the Kraft process,
the cooking liquor comprises a mixture of sodium hydroxide
and sodium sulfide. After the required cooking period,
softened and delignified wood chips are separated from the
cooking liquor to produce a fibrous mass of pulp. The
pulp produced by chemical pulping is called "brownstock."
2069447
The brownstock is typically washed to remove cooking
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, issued to Kirk 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
Delignificatipn in a Pipeline Reactor - A Pilot Study",
2069~47
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 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 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 Pulp and PaPer 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 7%
by weight of oven dry pulp.
Previously used high consistency oxygen delignifi-
cation processes have several disadvantages. In parti-
cular, 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 generally porous nature
of such mats. As a result of this uneven distribution,
certain areas of the high consistency mat, usually the
outer portions, are exposed to excessive amounts of the
alkaline solution. This excessive exposure is believed to
~5 cause nonselective degradation of the holocellulosic
2069447
materials resulting in a rela~ively weak pulp, at least
locally. On the other hand, other portions of the high
consistency mat, typically the inner 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 pulp
during a high consistency oxygen delignification process
wherein the oxygen delignified pulp has greater strength
and a lower lignin content than has been attainable by
prior art processes.
In accordance with the present invention, a
brownstock pulp is washed to an initial consistency. This
initial consistency of the pulp is then reduced to less
than about 10% by weight and preferably less than 5% by
weight to form a low consistency pulp. Alkaline material
is applied to the low consistency pulp by combining the
low consistency pulp with a quantity of alkaline material
in an aqueous alkaline solution in a manner to obtain a
substantially uniform distribution of the desired amount
of alkaline material throughout the pulp. This uniform
distribution of alkaline material is sufficient to assist
in the enhancement of delignification selectivity during
high consistency oxygen delignification compared to
processes where the alkaline material is only applied upon
high consistency pulp or is only applied at very low
amounts onto low consistency pulp.
To complete the application of the alkaline material
to the pulp, the consistency of the pulp is then increased
to at least about 18~ to form high consistency pulp. The
step of increasing the pulp consistency includes pressing
or otherwise processing the low consistency pulp in a
206~447
. .
manner to remove pressate containing alkaline material
while retaining the desired amount of alkaline material
distributed throughout the pulp.
A predetermined quantity of this pressate may be
retained in a holding tank, so that pressate may be
continuously returned or recycled directly to the alkaline
material combining step despite the intermittent or non-
continuous operation of the consistency increasing step.
All or at least a substantial portion (i.e., greater than
50~ and preferably about 75-95%) of this pressate is
directly recycled to the low consistency combining step.
The remaining pressate portion can be directed to the
brownstock pulp washer or to the plant recovery system to
maintain water balance in the mixing chest.
The amount of alkaline material to be retained upon
the high consistency pulp is at least about 0.8 to 7
percent by weight based on oven dry ('IOD") pulp, and
specifically between about 1.5 and 4 percent by weight for
southern softwood and between about 1 and 3.8 percent by
weight for hardwood. The pulp is then subjected to oxygen
delignification whereby enhanced delignification is
achieved.
The present invention also facilitates the pulp
bleaching processes that follow the high consistency
oxygen delignification of the alkaline material treated
pulp. These processes utilize less bleaching chemicals to
produce bleached paper products having superior strength
compared to paper products made according to conventional
high consistency pulp oxygen delignification processes.
Alternatively, the process enables one to achieve better
delignification selectivities, as evidenced by similar
lignin contents (i.e., K Nos. or Kappa numbers) with
higher strength (i.e., higher viscosities), after oxygen
delignification compared to conventionally treated pulp.
Also, the process of the invention enables one to achieve
2069~47
pulp which exhibits greatèr brightness compared to
conventionally treated pulps when exposed to the same
amount of bleaching chemical.
BRIEF DESCRIPTION OF THE DRAWINGS
~igure l is a schematic representation of the present
invention; and
Figure 2 is a graph showing the relationship between
pulp viscosity and K No. for softwood pulps treated with
alkaline material and delignified by oxygen according to
the invention compared to those representative of the
prior art.
DESCRIPTION OF PREFERRED EMBODIMENTS
The present invention provides high quality, high
strength, delignified wood pulp from Kraft pulp or pulps
produced by other chemical pulping processes. The
preferred starting material is unbleached brownstock 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 l, wood chips l 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 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 known
as Kraft cooking or the Kraft process and the pulp
produced by this process is known as Kraft pulp or Kraft
brownstock. Depending upon the lignocellulosic starting
~5 material, the, delignification results obtained with the
2069~47
conventional Kraft process may be increased by the use of
extended delignification techniques or the Kraft-AQ
process. Moreover, these techniques are preferred for
obtaining the greatest degree of reduction in K 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 OD weight of
the wood to be pulped, with amounts of from 0.02 to about
0.1% generally being 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 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 (not shown) in which the pulp 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.
~5
20~ 147
Brownstock 6 exiting blow tank 5 is directed to a
washer 7, which washes the pulp to a first consistency.
The washed pulp 8 is then introduced into a mixing chest 9
where it is substantially uniformly combined with
sufficient fresh l0 and recycle 14A alkaline material for
a time sufficient to distribute the desired amount of
alkaline material throughout the pulp. During this
treatment, the consistency of the brownstock pulp is
reduced and maintained at less than about 10% and
preferably less than about 5% by weight. The consistency
of the pulp is generally greater than about 0.5%, since
lesser consistencies are not economical to process in this
manner. A most preferred consistency range is 0.5 to
4.5%.
One skilled in the art can select the appropriate
quantities (i.e., concentrations and flow rates) of
alkaline solution and pulp treatment times in this step to
achieve a distribution of the desired amount of alkaline
material throughout the pulp. In particular, an aqueous
sodium hydroxide solution is combined with the low
consistency pulp in an amount sufficient to provide at
least about 0.8% to about 7% by weight of sodium hydroxide
on pulp based on oven dry pulp after the consistency
increasing step. A particularly useful sodium hydroxide
source is oxidized white liquor.
One skilled in the art would clearly recognize and
understand the difference between the "quantity" of
alkaline material utilized in or combined with the low
consistency pulp and the "amount" which is applied to or
is retained upon the pulp. To retain the desired amount
of alkaline material upon the pulp after the consistency
increasing step, a significantly larger quantity of
alkaline material must be combined with the low consis-
tency pulp in mixing chest 9. Thus, the quantity of
- 35 alkaline mat~rial which is utilized (i.e., present) in the
20694 17
mixing chest is always grèater~than the amount actually
applied to (i.e., retained within or upon) the pulp after
the consistency of the pulp is increased. Also, all
alkaline material is added to the pulp in mixing chest 9
to obtain a uniform dispersal of alkaline material in and
throughout the low consistency pulp which, after the
consistency increasing step, achieves the amount applied
to the pulp which is desired for high consistency oxygen
delignification of the pulp. The preferred amount of
alkaline material actually retained upon the high
consistency pulp will generally be between about 1.5 and
4% for southern softwood and between about 1 and 3.8% for
hardwood.
The low consistency mixing step includes uniformly
combining the brownstock pulp with an aqueous alkaline
solution for at least about 1 minute and preferably no
more than about 15 minutes. The mixing step is completed
when the aqueous alkaline solution is substantially
uniformly distributed throughout the low consistency pulp.
Treatment times of less than about 1 minute generally do
not provide sufficient time to attain a substantially
uniform distribution: this is typically attained after
about 10 to 15 minutes of mixing. Although continuing the
mixing for longer periods of time does not deleteriously
affect the process, no further benefit with respect to the
distribution of alkaline materials throughout the pulp is
obtained for longer mixing times, and equipment capacities
must be increased to provide longer residence times.
Such larger capacity equipment increases the capital cost
3Q for installation of the process.
The mixing step of the present invention can be
carried out over a wide range of temperature conditions.
A temperature range of from about room temperature to
about 150F is suitable, with temperatures ranging from
;
2069447
--10--
about 90F to about 150F being preferred. Standard
pressure or elevated pressure may be employed.
The quantity of aqueous alkaline solution present in
the mixing step of the present invention can vary greatly
according to the particular process parameters of the
delignification reaction; such variation in the amount of
aqueous alkaline material is within the scope of the
present invention. As will be appreciated by those
skilled in the art, the amount of alkaline solution
effective for the purpose of the present invention will
depend primarily upon the extent of delignification
desired in the subsequent oxygen bleaching step and the
strength of the particular solution being used. The
aqueous alkaline solutions of the present invention
preferably comprise a sodium hydroxide solution having a
concentration of from about 20 to about 120 g/1. This
solution is mixed with the low consistency pulp, so that
the overall mixture has concentration of alkaline material
of between 6.5 and 13.5 g/l, preferably around 9 g/1.
Thus, for a 5 to 15 minute treatment of 3 to 5 percent
consistency pulp at temperatures between 120 to 150 F at
these concentrations of alkaline material, a uniform
distribution of such alkaline material is obtained
throughout the pulp. According to preferred embodiments
2~ of the present invention, an aqueous sodium hydroxide
solution is added to the low consistency pulp in an amount
sufficient to provide from about 15 to about 30% by weight
of sodium hydroxide based on OD pulp weight.
The alkaline material treated pulp 11 is forwarded to
a thickening unit 12 where the consistency of the pulp is
increased, for example, by pressing to at least about 18%
by weight and preferably from about 25% to 35%. For the
preferred embodiment described above, the consistency is
increased to 29%; and the high consistency pulp 17 is
3~ directed to oxygen delignification reactor 20.
20~94~7
The pulp consistency increasing step also removes
residual liquid or pressate 13, which contains alkaline
material. To conserve chemical, this pressate is
recycled. When the consistency of the pulp 8 entering the
5 mixing chest 9 is on the same order (i.e., about equal or
slightly greater) as that of the high consistency pulp 17
which exits the thickener 12, the quantity of alkaline
material utilized in the combining step is minimized
because all pressate is advantageously directly recycled
back to the mixing chest 9, as shown in Figure 1 at 14A
and is retained within the low consistency pulp alkaline
treatment stage. Additional alkaline material 10 which is
needed to replace the amount which is applied to the pulp,
is added to mixing chest 9.
Optionally, a holding tank 16 may be included to
receive pressate 13. This holding tank 16 assists in the
smooth, continuous operation of the process by being able
to accumulate amounts of pressate 13 so as to provide an
uninterrupted flow of pressate containing alkaline
material to the mixing chest 9 despite intermittent or
non-continuous generation of pressate 13 from thickener
12. Thus, holding tank 16 provides a reservoir of
alkaline material which can be continuously directed to
mixing chest 9 for use in the low consistency pulp
alkaline treatment step. For example, this tank should be
sized at about 6000 cubic feet in order to have sufficient
volume to handle the pressate generated by the alkaline
treatment process for a 1000 air dried tons per day
("ADT/d") plant.
As noted above, brownstock 6 is washed in washer 7.
Although a conventional washer utilizing any appropriate
source of plant water can be utilized for washing
brownstock 6, it is advantageous to utilize a source of
wash water which is recycled from other steps in the
~5 process. Th~s, washer 7 is illustrated as including a
2069~47
,
split shower to receive wash water from separate
downstream sources.
A first portion 27 of the oxygen stage washer 23
filtrate 26 can be used to advantage by being recycled to
washer 7 to reduce the amount of water utilized by the
process. This filtrate portion 27 preferably passes
through a first shower at washer 7. A second shower
directs a portion 14B of the pressate 14 onto the pulp.
These portions 14B, 27 are used to wash the pulp 6, and to
recycle alkaline material onto the pulp as it is washed.
Most of the alkaline material in pressate portion 14B
becomes associated with the pulp and enters into mixing
chest 9. Washer effluent 15 is discharged to the plant
recovery system to maintain the water balance in the
mixing chest.
It is preferable to recycle pressate directed into
mixing chest 9 for use in the low consistency alkaline
treatment step, rather than to the second shower of washer
7. This avoids the possible loss of alkaline material to
the recovery system which would occur if the pressate 14
was introduced into the washer 7 due to "breakthrough"
into the effluent of the washer.
When the consistency of brownstock 6 entering washer
7 is on the same order as that exiting thickener 12, it is
possible to operate the process shown in Figure 1 with no
discharge of pressate from thickener 12. A closed system
is achieved, whereby all pressate is directly recycled to
mixing chest 9. The amount of alkaline material "lost"
due to retention upon the increased consistency pulp is
easily replaced by additional alkaline material 10 added
to the mixing chest 9 or holding tank 16. In this
arrangement, the quantities of alkaline material to be
utilized in the process would be minimized, since no
alkaline material is lost by intentional or unintentional
3~ discharge to,the plant recovery system.
2069 147
-13-
When the consistency of brownstock 6 entering washer
7 is lower than that of pulp 17 exiting thickener 12, a
buildup of liquid gradually occurs in the mixing chest 9
due to the recycle of pressate 14A. To remedy this
situation, a portion 14C of the pressate must be
discharged to the plant recovery system to maintain water
balance in mixing chest 9. Generally, a substantial
portion of greater than 50% and preferably about 75-95% of
pressate 14A is directly recycled to mixing chest 9 with
the remaining pressate portion being discharged at 14C to
the plant recovery system. Alternatively, the remaining
pressate portion may be directed to the split shower of
washer 7 via 14B.
The flow of pressate 14 can be divided so that
portion 14A is continuously directed to the mixing chest 9
while portion 14B is continuously directed to the washer 7
via the split shower. For this arrangement, pressate
portion 14A would again constitute at least 50% and
preferably, between about 75 and 95% of the total
pressate stream 14, with pressate portion 14B constituting
the balance. The wash filtrate 15 from washer 7 is then
discharged to the plant recovery system to maintain water
balance in the mixing chest 9. Also, the second portion
28 of the oxygen stage washer 23 filtrate 26 is discharged
to the plant recovery system.
The alkaline material containing pulp 17 is then
forwarded to the oxygen delignification reactor 20 where
it is contacted with gaseous oxygen 21 by any of a number
of well known methods. Suitable conditions for oxygen
delignification according to the present invention
comprise introducing gaseous oxygen 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 between the high consistency pulp
2069447
and the gaseous oxygen ra`nges-from about 15 minutes to
about 60 minutes.
After oxygen delignification in reactor 20, the
delignified pulp 22 is forwarded to a washing unit 23
wherein the pulp is washed with water 24 to remove any
dissolved organics and to produce high quality, low color
pulp 25. From here, pulp 25 can be 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 25. 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 described 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 present
invention is reduced by about 15 to 35% by weight compared
to the amount needed for the same starting pulp which is
not treated with alkaline material at low pulp
consistency. 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 not been uniformly combined with
alkaline material at low consistency. The amount of
alkaline material utilized in the extraction step would be
reduced by about 25 to 40% by weight for pulp treated with
alkaline material at low consistency as disclosed herein.
In addition to providing cost advantages with respect
to the reduced amounts of chemical necessary for such
2069~47
,
-15-
treatments, the process of thë 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
correspondingly reduced with similar savings in waste
water treatment facilities and related costs.
10 Examples
In order to illustrate the benefits and superior
performance of the methods of the present invention,
several tests were conducted utilizing the treatment
procedure depicted in Figure 1.
As the term is used herein, delignification
selectivity is a measure of cellulosic degradation
relative to the extent of lignin remaining in the pulp and
is an indication of the ability of the process to produce
a strong pulp with low lignin content. Differences in
delignification selectivity for oxygen delignification of
a particular pulp can be shown, for example, by comparing
the ratio of pulp viscosity to K No. or Kappa number. For
this invention, the lignin content of the pulp may be
measured by either K No. or Kappa number. One skilled in
the art can recognize the difference between these values
and can convert one number to the other, if desired.
Unless otherwise specified, 40 ml K Nos. will be reported.
The viscosity of a bleached pulp is representative of
the degree of polymerization of the cellulose in the
bleached pulp and as such is representative of the pulp.
On the other hand, K No. represents the amount of lignin
remaining in the pulp. Accordingly, an oxygen
delignification reaction that has a high selectivity
produces a bleached pulp of high strength (i.e., high
viscosity) and low lignin content (i.e., low K No.).
2069447
-16-
Example 1 (Prior art high consistency pulp alkaline
treatment)
Southern pine Kraft brownstock having a K No. of
about 24 (Kappa number of 30.9) was pressed without
alkaline solution treatment to a consistency of about 30-
36% by weight to produce a high consistency mat of
brownstock. The mat of brownstock was sprayed with a 10%
sodium hydroxide solution in an amount sufficient to
produce approximately 2.5 weight percent sodium hydroxide
based on pulp dry weight. Dilution water was added in an
amount sufficient to adjust the brownstock mat to about
27% consistency. The high consistency brownstock mat was
then subjected to oxygen delignification using the
following conditions: 110 C, 30 minutes, 80 psig 2- The
oxygen delignified pulp produced according to this
procedure was tested and found to have a K No. of 13
(Kappa number of 15.2) and a CED viscosity of about 14.8
cps. This oxygen delignified pulp was further bleached by
known technology. The strength and physical properties of
both the oxygen delignified pulp and the fully bleached
pulp are shown in Tables 1 and 2, respectively.
TABLE 1
Comparison of Oxygen Stage Delignification Results
25on Pulps Produced by Example 1 and Example 2
EXAMPLE 1 EXAMPLE 2
K No. 13 9
Viscosity (cps) 14.8 14.0
Ratio of 1.14 1.55
Viscosity\K No.
2069447
TABLE 2
Comparison of Fully Bleached Strength Properties
of PulPs Produced by Example 1 and Example 2
EXAMPLE 1 EXAMPLE 2
Final G.E.
brightness, % 83 83
C.S. Free-Breaking Tear Breaking Tear
ness, ml.Length-km Factor, Dm2 Length-km Factor, Dm2
6586.42 55.7 7.00 55.5
5168.25 73.6 8.35 67.4
3378.80 74.1 9.07 71.8
Bleaching of the oxygen delignified pulp was
conducted in three stages: chlorine, caustic extraction
and chlorine dioxide. The final bleached pulp of 83 G.E.
brightness was obtained using the bleaching and extraction
conditions of Table 3 and the chemical charges (percent
based on OD pulp) listed in Table 4. Also, the pulps were
well washed between bleaching stages.
3~
2069'~47
-18-
TABLE 3
Bleaching Conditions in the Chlorine, Extraction and
Chlorine Dioxide Staqes for Example 1 and ExamPle 2
Chlorine Staqe
Time, min. 15
Temperature, o C 50
Consistency, % 3
Extraction Stage
Time, min. 60
Temperature, C70
Consistency, %12
Chlorine Dioxide Stage
Time, min. 120
Temperature, C60
Consistency, %12
TABLE 4
Bleach Chemical Usage in Chlorine,
Extraction and Chlorine Dioxide Stages
EXAMPLE 1 EXAMPLE 2
Chlorine Staqe
Chlorine, % on fiber 3.6 2.4
Chlorine Dioxide, % 0.6 0.4
Extraction Staqe
Sodium Hydroxide, ~ 1.5 1.5
Chlorine Dioxide Staqe
Chlorine Dioxide, ~ 0.28 0.23
2069~7
--19--
ExamPles 2-5 (Low consistency pulp alkaline treatment)
Examples 2-5 illustrate the benefits in degree of
delignification and delignification selectivities obtained
during high consistency oxygen delignification for pulps
which are treated with alkaline material only at low
consistency.
Example 2
The same pine Kraft brownstock as used in Example 1
was introduced into a mixing chest, such as 9 of Figure 1.
Sufficient dilution water was added to obtain a brownstock
consistency of about 3% by weight in the mixing chest. A
sufficient volume of 10% NaOH solution was added to effect
a 30% NaOH addition based on OD pulp. The brownstock and
the aqueous sodium hydroxide solution were uniformly mixed
at room temperature for about 15 minutes to combine the
alkaline material with the brownstock. The resulting
alkaline material containing brownstock was then pressed
to a consistency of about 27% by weight. After pressing,
the sodium hydroxide on the fiber equaled about 2.5%, as
in Example 1. The alkaline material treated brownstock
was then bleached according to the oxygen delignification
procedure described in Example 1. The oxygen delignified
pulp was then washed to remove organics. The resulting
oxygen stage pulp had a K No. of 9 (Kappa number of 10.8)
and a CED viscosity of 14Ø The oxygen bleached pulp was
further bleached by known technology at the conditions
shown in Example 1. The properties of the oxygen
delignified pulp and the fully bleached pulp of this
Example are also shown above in Tables 1 and 2,
respectively.
As can be seen from a comparison of Examples 1 and 2,
the procedure of Example 2 produced an oxygen delignified
pulp having greater delignification (lower K No.) at about
the same visçosity than the prior art method of Example 1
2~S9~ 17
-20-
which applies all the alkaline~ material upon the high
consistency pulp. Furthermore, utilizing a low
consistency alkaline treatment of pulp in accordance with
Example 2 provides enhanced delignification without
significant change in strength properties. Thus,
increased delignification selectivity is achieved.
As a result of the lower K Nos. of pulp produced by
Example 2, subsequent bleaching steps can be adjusted to
accommodate the higher delignified pulp. Thus, the
bleaching stages for such pulp require less bleaching
agents (as shown in Table 4) or shorter bleaching times
than for pulp which is not treated with alkaline material
at low consistency.
Example 3
Pulp produced from softwood (pine) in a process
similar to that of Example 2 is compared to that produced
conventionally (i.e. with no low consistency alkaline
treatment step) as in Example 1. The average sodium
hydroxide dosage applied only to high consistency pulp for
subsequent oxygen delignification of the pulp was found to
be 45 pounds per oven dried ton (lb/t) or 2.3%. At that
level, the average reduction in K No. across the oxygen
delignification reactor was 10 units. For the same level
of sodium hydroxide applied only to the low consistency
pulp prior to high consistency oxygen delignification, an
average K No. drop during delignification was found to be
13 units: a 30% increase compared to the prior art.
The average K No. and viscosity for conventional pulp
was 12.1 and 14.4 cps, respectively. For the low
consistency alkaline material treatment process, the
average K No. at essentially the same viscosity (14.0 cps)
was 8.3, an increase in delignification selectivity of
about 41% (1.69 vs. 1.19), as shown in Table 5.
~5
20~4 17
-21-
Bleach plant response for pulps prepared according to
the above low consistency alkaline treatment process was
compared to that for pulps prepared conventionally and is
shown below in Table 5.
s
2069447
-22-
TABLE 5
Pulp Property and Bleach Chemical Comparison
(Pine~
Low Consistency
Conventional Treated
Diqester
K No. 21.9 20.5
Viscosity (cps)21.5 20.5
Ratio of .98 1.0
Viscosity/K No.
2 Delignification Staqe
K No. 12.1 8.3
Viscosity (cps)14.4 14.0
Ratio of 1.19 1.69
Viscosity/K No.
Caustic, lb/t 39.4 46.0
Delignification (%) 44.7 59.5
Bleach Plant
Chlorine/Chlorine Dioxide Stage
Cl2, lb/t 51.2 34.4
Cl02, lb/t 7.0 4.6
Tot. Act. Cl, lb/t 69.4 46.4
Extraction Staqe
NaOH, lb./t 35.2 23.8
Chlorine Dioxide Bleach Staqe
Cl02, lb/t 10.6 9.0
Viscosity (cps) 12.6 11.9
Dirt 5.6 2.5
~5
. ~ ~ 9 1 ~ ~
-23-
Table 5 illustrates that total active chlorine usage
in the next stage of bleaching was reduced by about 1/3
(i.e., 69.4 pounds per ton vs. 46.4 pounds per ton). In
addition, sodium hydroxide requirements for the extraction
stage were also reduced by about 1/3 (24 lb/t vs. 35
lb/t). Chlorine dioxide in the final bleaching stage was
reduced by about 1/6 (9 lb/t vs. 10.6 lb/t).
Example 4
Comparison tests similar to Example 3 were carried
out for hardwood pulp. Again, it was found that a
significantly larger K No. drop during the oxygen
delignification reaction is achieved using a treatment
process where alkaline material is applied only to low
consistency pulp compared to conventional processing. The
sodium hydroxide dosage for oxygen delignification of
hardwood is 27 lb/t, or 1.4%. A K No. drop of about 5
units during the delignification step was obtained for the
conventional process. For the same level of sodium
hydroxide utilized according to the above low consistency
process, an average K No. drop of about 7.3 units was
obtained, an increase of almost 50%.
The average hardwood K No. and viscosity were found
to be 7.6 and 16 cps, respectively. For the above low
consistency treatment, a K No. of 6 and a viscosity of
17.7 was obtained. Also, the K No. at the same viscosity
as the prior art alkaline material treated pulp (16 cps),
was found to be 5.8. An increase of delignification
selectivity of about 40% (2.95 vs. 2.10) is achieved, as
shown in Table 6.
Delignification selectivity can also be expressed in
terms of the change in viscosity versus the change in K
No. between brownstock and delignified pulps. In
comparing pulps which are treated with alkaline material
only at low consistency to those of the prior art, there
2a6sl47
-24-
is a greater increase in delig~nification selectivity for
increased degrees of delignification. For a change in K
No. of 4 units, the average change in viscosity was 4 cps
for pulps produced by the conventional process. By
contrast, the change in K No. for the same change in
viscosity for pulps produced by the low consistency pulp
treatment was 7 units. Expressed in terms of a
selectivity ratio, the selectivity for the low consistency
treated pulp was 1.75 and that for the conventional
process was 1 (cps/K No.), an increase of about 75%.
A comparison of bleach plant response of oxygen
delignified pulps prepared using the above low consistency
alkaline material treatment in terms of bleach chemical
application is compared to conventionally prepared oxygen
delignified pulps in Table 6.
2069~47
-25-
TABLE 6
Pulp Property and Bleach Chemical Comparison
(Hardwood)
Low Consistency
Conventional Treated
Diqester
K No. 12.3 13.0
Viscosity (cps)21.6 23.4
Ratio of 1.75 1.80
Viscosity/K No.
2 Deliqnification Stage
K No. 7.6 6.0
Viscosity (cps)16.0 17.7
Ratio of 2.10 2.95
Viscosity/K No.
Caustic, lb/t27.6 26.4
Delignification (%) 38.0 54.0
Bleach Plant
Chlorine/Chlorine Dioxide Stage
Cl2, lb/t 27.0 22.7
Cl02, lb/t 5.6 4.7
Tot. Act. C1, lb/t 41.6 34.9
Extraction Stage
NaOH, lb./t 18.9 13.3
Chlorine Dioxide Bleach Stage
Cl02, lb/t 5.5 4.7
Viscosity (cps)14.6 14.9
Dirt 32.0 34.0
2069~47
-26-
Table 6 illustrates that total active chlorine usage
in the chlorine stage was reduced by about 1/6 (i.e., 34.9
lb/t compared to 41.6 lb/t), while caustic requirements
for the extraction stage were reduced by more than 29
(i.e., 13.3 lb/t vs. 18.9 lb/t) compared to prior art
pulp. The chlorine dioxide in the final bleaching stage
was reduced by more than 14~ (i.e., 4.7 lb/t vs. 5.5
lb/t). The final pulp properties with regard to viscosity
and dirt values were essentially the same.
Example 5
To illustrate the effect of low consistency alkaline
material treatment on pulp prior to oxygen delignification
and its contribution to the overall effectiveness of Kappa
drop and total yield, the Kappa number and yield were
determined for both conventional and low Kappa number
Kraft/AQ brownstocks. The results are presented in Table
7.
TABLE 7
LOW CONSISTENCY OXYGEN
ALKALINE TREATMENT DELIGNIFICATION
Initial Final Final
Time Kappa Kappa Yield Kappa Yield Viscosity
Brownstock (Min.) Number Number (%) Number (%) (CPS)
25 ~Conven. 5 28.1 26.5 99.5 12.0 95.2 14.
2Conven. 15 28.1 27.5 98.7 13.4 96.3 15.1
3K/AQ 5 21.6 20.3 100.0 8.9 96.7 15.2
4K/AQ 5 21.6 -- -- 8.1 9'7.2 13.9
' 2.4% NaOH
2 2.19'o NaOH
30 3 2.1 % NaOH
2.6% NaOH
For a conventional kraft brownstock having a Kappa
number of 28.1 treated with sodium hydroxide (2.4% on pulp
after pressing) at 3% consistency for 5 minutes, the
2a~9~47
-27-
starting Kappa number decreased 1.6 units to a post
treated Kappa number of 26.5. This represented a 9.6%
contribution to the total Kappa number drop experienced
following alkaline treatment and oxygen delignification
(Kappa number of 12.0). The yield across the low
consistency alkaline treatment stage was 99.5%.
Approximately half of the 0.5% loss in yield can be
attributed to loss of lignin with the remainder due to a
loss in carbohydrates. The total yield after oxygen
delignification was 95.2%.
The same starting brownstock was treated with sodium
hydroxide (2.1% on pulp after pressing) at 3% consistency
for 15 minutes. The starting Kappa number decreased 0.6
units to a Kappa number of 27.5. This represented a 4.2%
contribution to the total Kappa number drop experienced
following low consistency alkaline treatment and oxygen
delignification (Kappa number of 13.4). The yield across
the alkaline treatment stage was 98.7%.
For a low Kappa number kraft/AQ brownstock treated
with sodium hydroxide (2.11% on pulp after pressing) at 3%
consistency for 5 minutes, the Kappa number decreased 1.3
units to 20.3. This Kappa number drop represented a 10%
contribution to the total Kappa number drop experienced
following oxygen delignification (Kappa number of 8.9).
There was essentially no yield loss detected across the
alkaline treatment stage. The total yield loss following
oxygen delignification was 96.7%. A second oxygen
delignification of the same kraft/AQ starting brownstock
resulted in a similar Kappa number of 8.1 and yield of
97.2%.
This Example 5 shows that no significant amount of
delignification occurs during the low consistency alkaline
treatment of the pulp. This example also shows that there
is no significance to the time of treatment with alkaline
material at low consistency up to about 15 minutes. As is
2~6~944 7
-28-
further shown by Examples 2-5, however, the low
consistency alkaline treatment does significantly increase
the relative amount of delignification obtained during
subsequent high consistency oxygen delignification step as
compared to pulps treated in the conventional manner.
This example also shows that the process is effective with
a low Kappa number brownstock in taking the pulp to a very
low Kappa number without any significant decrease in
viscosity.
The uniform distribution of the alkaline material
throughout the pulp during the low consistency combining
step ensures that the pulp fibers are more optimally
associated with the alkaline material than is otherwise
possible according to prior techniques. This results in
enhanced delignification selectivity during subsequent
oxygen delignification in that the delignified brownstocks
have strength and degrees of delignification that are
generally superior to those attainable by the prior art.
Also, the delignification selectivity of the oxygen
delignification reaction is unexpectedly improved.
When alkaline material is applied only to high
consistency pulp as in the prior art, a delignification
(i.e., reduction in K No.) of up to 50% can be achieved
without substantially damaging the cellulose portions (and
thus without substantially reducing the strength) of the
pulp. In the present invention, it is possible to obtain
a reduction in K No. for the incoming pulp of greater than
50% and generally at least about 60% during oxygen
delignification with essentially no damage to the
cellulose portion of the pulp. Reductions of 70% and more
can be achieved, if desired.
For example, upon entering the oxygen delignification
stage, pulp K Nos. for the particular pulp range from
about lO to 26, depending upon the type of wood and type
of pulping c~nducted upon the particular wood. After
20S9~47
delignification, the K No. is reduced to about 5 to lo.
For softwood pulp, K Nos. generally range from 20-24
(target of 21) prior to delignification, while after
delignification, the K Nos. are in the range of 8-10. For
hardwood pulp, K Nos. of 10-14 (target 12.5) prior to
delignification and K Nos. of 5-7 after delignification
are generally obtained by the present process.
For either type of pulp, the viscosity prior to
delignification is generally about 19 or greater, while
after delignification is above about 13 (generally 14 or
above for softwood and 15 or above for hardwood).
Typically, this change in viscosity from before to after
delignification would be about 6 cps. or less. Moreover,
it has been found that the change in viscosity per change
in K No. is a constant for decreases in K No. up to about
17 units.
Thus, delignification selectivity is enhanced by the
low consistency alkaline material combining step, with an
increase of at least 20% in delignification compared to
prior art delignification processes. The avoidance of
deterioration of the cellulose component of the pulp is
evident by the minimal change in viscosity of pulp from
before to after oxygen delignification.
Example 6
The data presented in Examples 2-5, along with
numerous other predicted and observed values, have been
compiled for softwood pulp in graphical form in Figure 2.
Figure 2 also includes curves generated from combined data
from actual tests, and numerous other predicted and
observed results, which illustrates the relationship of
viscosity to K No. for softwood from the prior art pulp
treatment process of Example 1.
As shown in Figure 2, the prior art process of
~5 Example 1 achieves typical pulp properties after oxygen
20~9417
-30-
delignification defined by the curve labeled Prior Art.
It is desirable to maintain pulp strength, as measured by
viscosity, at higher viscosity levels, while achieving
effective delignification as measured by a decrease in K
No. Figure 2 illustrates that enhanced delignification
(lower K Nos.) may be attained at a given viscosity value
according to the curve representing the invention, for a
low consistency pulp alkaline material treatment as
compared to the lesser delignification and viscosity
values according to the Prior Art curve.
Example 7
The following laboratory tests are included to
further illustrate how to achieve a uniform distribution
of alkaline material upon the pulp in accordance with the
process of the present invention.
An unbleached brownstock pine pulp was prepared
having a K No. of 19.54 and a viscosity of 24.9. Two
samples of this pulp at a consistency of 7.7% were treated
with 3% NaOH at a temperature of 60C for 1 and 15
minutes, respectively. Thereafter, the consistency of the
pulp was increased to 27% and the NaOH content of the pulp
was found to be about 0.67%. This pulp was directed to an
oxygen delignification reactor at a pressure of 80 psi and
a temperature of 110C for 30 minutes without the further
addition of alkaline material.
Next, two additional samples of the unbleached pulp,
each at a consistency of 3%, were treated with a NaOH
application of about 35% at a temperature of 60C for 1
and 15 minutes, respectively. Thereafter, the consistency
of the pulp was increased to 27%, while retaining a NaOH
content of 3% throughout the pulp, and the pulp was
directed to an oxygen delignification at a pressure of 80
psi and a temperature of 110C for 30 minutes without the
~5
20~94~7
further addition of alkaline material. The results are
shown in Table 8 below:
TABLE 8
Properties after
Oxy~en Deli~nification
C,,,.~ .y Mixing Time K No. Viscosity
Sample % (minutes) (25 ml) (cps)
A 7.7 1 17.3723.2
B 7.7 1 17.4322.6
C 7.7 15 17.7724.3
D 7.7 15 17.3422.0
E 3.0 1 8.7414.8
F 3.0 1 8.3414.8
G 3.0 15 8.2415.3
H 3.0 15 8.7314.3
The treated pulp of samples E-H retains a much
1 5 greater amount (i.e., 396) of sodium hydroxide than that of
samples A-D, because a much larger quantity of sodium
hydroxide is mixed with the pulp. Samples E-H show a
decrease in K No. of the pulp of at least about 55.3%,
while the K No. decrease of Samples A-D is much smaller
20 and is, at best, about 11.3%. Thus, the samples (E-H)
treated in accordance with the process of the present
invention increases delignification by about 49.6% over
the comparative samples.
For the same unbleached brownstock pulp of this
25 example, the preceding tests were repeated with the
following changes:
Modification Modification
1 2
1st Stage: NaOH, % on pulp 3 24
Consistency, % 3.5 3
Temperature, ~C 48 48
Oxygen stage: NaOH, % on pulp 0.44 3
Consistency, ~ 20 20
The NaOH treatment time for each modification was
conducted both at 2 minutes and 15 minutes. As noted, the
20~9~7
consistencies of the unbleachëd pulp were essentially the
same (3.5% vs. 3%). Results are shown in Table 9.
TABLE 9
Properties after
Oxy~en Deli~nification
ConsistencyMixing Time K No. Viscosity GE
Sample - % (minutes) (25 ml) (cps) Bn~htness
3.5 2 15.75 23.4 24.8
J 3.5 2 15.34 22.4 25.2
K 3.5 15 14.78 22.6 25.9
L 3.5 15 15.00 22.7 25.5
1 0 M 3.0 2 8.59 13.3 36.6
N 3.0 2 8.29 14.2 35.3
O 3.0 15 8.14 13.1 36.3
P 3.0 15 8.44 13.8 36.5
Due to the increased amount of NaOH mixed with the
low consistency pulp, a much greater amount of NaOH is
retained on the high consistency pulp. Due to this
increased amount of NaOH, samples M-P achieve a decrease
in K No. of at least about 56%, while samples I-L, at
best, achieve a decrease of only about 24.4%. Again, the
samples (M-P) prepared by the present process obtain
increased delignification by at least 41.9% compared to
the comparative samples. As noted above, this is due to
the increased amounts of sodium hydroxide retained upon
the high consistency pulp due to the uniform mixing and
distribution of appropriate amounts of sodium hydroxide
throughout the low consistency pulp.
While it is apparent that the invention herein
disclosed is well calculated to fulfill the objectives
stated above, it will be appreciated that numerous
modifications and embodiments may be devised by those
skilled in the art. 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.
