U.S. patent application number 10/657956 was filed with the patent office on 2005-03-10 for extended retention and medium consistency pulp treatment.
Invention is credited to Yin, Caifang.
Application Number | 20050051288 10/657956 |
Document ID | / |
Family ID | 34226677 |
Filed Date | 2005-03-10 |
United States Patent
Application |
20050051288 |
Kind Code |
A1 |
Yin, Caifang |
March 10, 2005 |
Extended retention and medium consistency pulp treatment
Abstract
Treatment of digested or O2 delignified cellulosic pulp in a
chlorine dioxide (D.sub.o) delignification stage, preceding
bleaching of the pulp, at a medium pulp consistency for an extended
period of time. Unexpected benefits include improvement in the
delignification efficiency and selectivity, a lowering of the
chlorine dioxide consumption in the overall post-digestion or
post-O2 delignification and bleaching process, a reduction in the
filtrate volume, a reduction of COD/AOX from an ECF bleaching
plant, improved pulp properties, and enhanced removal of Hex-A. The
process may be implemented in existing three, four or five-stage
bleaching plants.
Inventors: |
Yin, Caifang; (Mason,
OH) |
Correspondence
Address: |
INTERNATIONAL PAPER COMPANY
6285 TRI-RIDGE BOULEVARD
LOVELAND
OH
45140
US
|
Family ID: |
34226677 |
Appl. No.: |
10/657956 |
Filed: |
September 9, 2003 |
Current U.S.
Class: |
162/65 ; 162/67;
162/76; 162/88; 162/89; 162/90 |
Current CPC
Class: |
D21C 9/123 20130101;
D21C 9/1057 20130101; D21C 9/1005 20130101 |
Class at
Publication: |
162/065 ;
162/067; 162/076; 162/088; 162/089; 162/090 |
International
Class: |
D21C 003/18 |
Claims
1. In a process for the treatment of digested cellulosic pulp
preparatory to bleaching of the pulp, the improvement comprising
subjecting the pulp at a medium consistency to chlorine dioxide for
a time period of at least 60 minutes to delignify the pulp prior to
bleaching thereof.
2. The improvement of claim 1 wherein the pulp at a medium
consistency is subjected to chlorine dioxide for a time period
between about 60 minutes and about 180 minutes.
3. The improvement of claim 1 wherein the pulp is at a consistency
of between about 10% and about 15%, based on the weight of oven
dried pulp.
4. The improvement of claim 1 wherein the pulp is prewashed
following digestion thereof and prior to subjection of the pulp to
chlorine dioxide.
5. The improvement of claim 1 and including the step of subjecting
said digested pulp to O2 delignification prior to subjecting said
pulp to chlorine dioxide.
6. The improvement of claim 1 wherein said chlorine dioxide is in
either a liquid or gaseous state.
7. The improvement of claim 1 wherein said pulp comprises either
hardwood or softwood pulp.
8. The improvement of claim 1 and including the further step of
subjecting said first treated pulp to one or more bleaching
operations for enhancing at least the brightness of the pulp and
removal of dirt from the pulp.
9. The improvement of claim 8 wherein said bleaching operations
include sequentially subjecting the first treated pulp to an
extraction which includes oxygen, peroxide or a combination of the
same.
10. The improvement of claim 9 wherein said extraction is followed
by one or more exposures of the pulp to chlorine dioxide.
11. The improvement of claim 1 wherein said step of subjecting of
the pulp at a medium consistency for said time period effects
substantial removal of hexauronic acid from the pulp.
12. The improvement of claim 10 wherein at least 50 and about 80%
of the hexauronic acid originally in the pulp is removed.
13. In a process for preparation of a digested cellulosic pulp for
use in a papermaking process, the improvement comprising the steps
of subjecting the pulp at a consistency between about 10% and about
15% based on the weight of oven dried pulp in a vessel to chlorine
dioxide for a time period of at least 60 minutes, thereafter
subjecting this first-treated pulp to a bleaching sequence which
includes at least one stage in which the pulp is subjected to
chlorine dioxide for a time period sufficient to produce a pulp of
a desired brightness and viscosity, and thereafter recovering the
pulp for use in a papermaking process.
14. A sequence for preparation of digested cellulosic pulp for use
in papermaking comprising D.sub.emc followed by one or more of
either E.sub.o, E.sub.p, Eop, D.sub.1 or D.sub.2.
15. The sequence of claim 13 wherein said D.sub.emc is followed by
E.sub.op and D.sub.1.
16. The sequence of claim 14 and including D.sub.2 following
D.sub.1.
17. A method for retrofitting a preexisting multi-stage digested
cellulosic pulp treatment facility comprising the step of
incorporating into said facility a D.sub.emc first stage.
18. The method of claim 16 wherein the preexisting multistage
facility includes a D.sub.o stage and said D.sub.emc stage
supplants said D.sub.o stage.
19. The method of claim 17 wherein said preexisting multistage
facility includes a D.sub.1 stage and said D.sub.emc stage
supplants said D.sub.1 stage.
20. The method of claim 17 wherein said preexisting multistage
facility includes a D.sub.2 stage and said D.sub.emc stage
supplants said D.sub.2 stage.
21. A method for the removal of hexauronic acid from a digested
cellulosic pulp comprising contacting a medium consistency pulp
containing hexauronic acid in a vessel with chlorine dioxide for a
time sufficient to extract said hexauronic acid from said pulp,
said time being not less than about 75 minutes.
22. The method of claim 21 wherein said pulp in said vessel is of a
pH of between about 2 and about 4 and at a temperature of between
about 100 and about 170 degrees F.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] Not Applicable
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] Not Applicable
BACKGROUND OF THE INVENTION
[0003] This invention relates to the processing of cellulosic
fibrous material in preparation for use in papermaking. In
particular, the invention is directed to a process for enhancing
the delignification of cellulosic fibrous material preparatory to
(especially ECF) bleaching of the pulp.
[0004] In the papermaking industry it is common practice to digest
chips or like particulates of a cellulosic fibrous material, such
as wood chips, with a view to separating the cellulosic fibers from
the lignin which binds the fibers to one another and to produce a
pulp. This process step is commonly termed "digestion" and is a
major step in "delignification" of the pulp. Various chemicals and
various process steps are employed in the digestion of different
types of chips. Particularly, softwood and hardwood chips are
employed as starting materials in a papermaking process. Lignin is
responsible for the brown coloration of paper. Kappa number is an
index used by the pulp and paper industry to express the lignin
content of a pulp. Many mills employ oxygen (O2) for further
delignification of pulp before ECF bleaching, Excessive removal of
lignin (delignification) in digestion and/or O2 delignification
operations tend to degrade the mechanical strength of the
cellulosic fibers.
[0005] The output from a digester contains the liquor employed in
the digestion process, individual separated fibers, clumps of
fibers, those materials which are the byproducts of the digestion
of the lignin such as resins, phenolics, nonphenolics, hexenuronic
acid ("Hex-A"), trash, and other matter (collectively, "brown
stock").
[0006] Depending upon the intended end use of the paper to be made
from the pulp, the brown stock may be washed, and/or O2 delignified
(for some mills) followed by bleaching to enhance various of its
properties, such as brightness, dirt removal, etc. Bleaching
commonly is carried out in stages, each stage being designed to
effect one or more desirable enhancements of the pulp. Three to
five stages are common in a bleach plant. The first two stages
(D.sub.oE.sub.OP) of current employed in bleach plants are also
delignification stages. As is recognized in the art, each stage
comprises a reaction tower and each tower is preceded by a washer.
In the present instance, when identifying stages of a bleach plant
washers are at times, merely assumed.
[0007] In typical multi-stage elemental chlorine free (ECF) bleach
plants, the first chlorine dioxide stage (D.sub.o) is for
delignification and the other following stages (D.sub.1 and D.sub.2
for example) are for bleaching (brightness development, dirt
removal for pulp cleanness. Pulp (fiber) strength is a factor in
all stages. Because of this difference in objectives of these
stages, the D.sub.o stage of a typical current bleach plant is
operated at significantly different operating parameters from the
operating parameters of the remaining following stages. Pertinent
ones of these operational parameters for currently existing bleach
plants as given in Table I below:
1 TABLE I D.sub.o D.sub.1 or D.sub.2 Consistency, % 3-4 10-12
Retention time, min. 25-45 (<60) 120-180 Steam consumption no
yes Temperature, .degree. C. 50-65 65-80 pH 2-3 3-4
[0008] In past years, the first stage, D.sub.o, of a bleach plant
used elemental chlorine at low pulp consistency and short retention
time within a reaction vessel (tower) for delignification because
of high reactivity of elemental chlorine toward both phenolic and
nonphenolic lignin. In converting the traditional elemental
chlorine-based bleach plants to elemental chlorine-free (ECF)
bleach plants to, among other things, eliminate the formation of
dioxins produced in elemental chlorine-based pulp bleaching
operations, a majority of the existing mills simply converted the
old chlorine (C or C.sub.1) stage to a chlorine dioxide (D.sub.o)
stage, using the existing tower equipment. This practice is largely
based on the conventional wisdom that chlorine dioxide reactions
with lignin are fast and in the D.sub.o stage, 90% delignification
of the pulp is completed within 10 minutes of contact between the
chlorine dioxide and the pulp.
[0009] The industrial experience with conversion of the old C or
C.sub.d stage to a D.sub.o stage, however, has shown a significant
decrease in the delignification efficiency as measured by higher
chlorine/extraction (CE) or chlorine/extraction with oxygen
(E.sub.o) and/or peroxide (CEop) Kappa at a similar or higher
active chlorine charge or kappa factor (% active chlorine/kappa)
and increased bleaching cost by as much as 20% over elemental
chlorine bleaching. Because of much lower reactivity of chlorine
dioxide than elemental chlorine toward lignin (particularly
nonphenolic lignin) and other impurities, the available retention
time and consistency with the old chlorination (C) tower, when used
with a chlorine dioxide stage is suboptimal for the D.sub.o stage
operation.
[0010] Further, in the D.sub.o stage, it has been noted that only
about 10-30% of the hexenuronic acid (Hex-A) in the pulp is removed
under the operating conditions called for in the D.sub.o stage of
the current elemental chlorine free (ECF) bleach plant. The lower
Hex-A removal efficiency in the current D.sub.o stage is a result
of both lower chlorine dioxide reaction efficiency and the absence
of the required temperature and retention time for the acid
hydrolysis of Hex-A. Hex-A content is an issue particularly for
those hardwood species such as eucalyptus, maple, and birch. Hex-A
also plays key roles in pulp bleaching chemical consumption, AOX
and COD formation, oxalate related scale, and pulp brightness
stability. It is known that the use of an acid removal (A) of Hex-A
does not remove lignin, hence an A stage does not contribute to
delignification of the pulp.
[0011] As noted, existing bleach plants are not readily convertible
to new or different processing conditions which alter the
throughput of the pulp being processed and/or affect the follow-on
bleaching stages. Such alterations are particularly troublesome if
they require costly modification of the existing processing
equipment. As a result retrofitting of an existing bleach plant to
accommodate different or modified stages of handling of the pulp
from the digester through the bleaching operation have heretofore
not been feasible from either an operational or an economic
standpoint. For example, installation of a new stage in a bleach
plant can cost more than $3 million for an existing ECF bleach
plant and therefore is particularly challenging to a
capital-limited mature paper industry. In similar manner, deleting
a stage from an existing bleach plant for practicing new technology
can be a particularly "hard-sell" to a bleaching mill in that the
number of stages (and their operating parameters) are well
established and proven to produce a pulp having certain
characteristics, such as brightness, etc. and any change to this
established process normally requires strong reason to make such
change.
BRIEF SUMMARY OF THE INVENTION
[0012] In the course of efforts to develop more efficient ECF
bleaching technology for cost reduction, the present inventor
unexpectedly discovered that if the consistency of the pulp leaving
the washer (following the digester) is adjusted to a medium
consistency and the time of residence of the pulp in a follow-on
delignification stage preceding bleaching stages is extended
(D.sub.emc) (herein at times referred to as the "extended medium
consistency" or "D.sub.emc" technology"), there occurs an
unexpected improvement in the delignification efficiency and
selectivity, and a lowering of the chlorine dioxide consumption in
the overall post-digestion delignification and bleaching process, a
reduction of the filtrate volume, a reduction of COD (chemical
oxygen demand) and AOX (absorbable organic halides) emissions from
the ECF bleaching, decreased oxalate-related scale in ECF bleach
plants, improved pulp properties, and an increase in Hex-A removal
efficiency over the current short retention low consistency (LC)
D.sub.o or D.sub.LC stage. Moreover, the inventor has found that
the D.sub.emc technology may be implemented in, and its benefits
realized with, existing bleach plants with a minimum of cost and
change to the existing bleach plants. The present invention may be
employed with pulp which has only been washed after leaving the
digester or with pulp which has passed through an O.sub.2 treatment
between the digester and the D.sub.EMC stage.
[0013] Cutting a bleach plant stage for practicing the D.sub.emc
technology can be a particularly "hard-sell" to those mills with
four-stage bleach plants such as DEopDD or DEopDP. Accordingly,
among other things, the present invention provides for existing
bleach plants to practice D.sub.emc delignification technology for
the benefits, in some instances eliminating a bleaching stage or in
other instances without cutting the number of bleach stages, all at
minimum capital expenditure.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0014] FIG. 1 is a schematic flow diagram of a typical prior art
digester/bleaching operation as employed in papermaking;
[0015] FIG. 2 is a schematic diagram including flow patterns of a
prior art five-stage bleaching plant;
[0016] FIG. 3 is a schematic diagram including flow patterns of a
first optional retrofit of the five-stage bleaching plant depicted
in FIG. 2;
[0017] FIG. 4 is a schematic diagram including flow patterns of a
second optional retrofit of the five-stage bleaching plant depicted
in FIG. 2;
[0018] FIG. 5 is a schematic diagram including flow patterns of a
third optional retrofit of the five-stage bleaching plant depicted
in FIG. 2;
[0019] FIG. 6 is a schematic diagram including flow patterns of a
fourth optional retrofit of the five-stage bleaching plant depicted
in FIG. 6.
[0020] FIG. 7 is a schematic diagram including flow patterns of a
prior art four-stage bleaching plant;
[0021] FIG. 8 is a schematic diagram including flow patterns of a
first optional retrofit of the four-stage bleaching plant depicted
in FIG. 7;
[0022] FIG. 9 is a schematic diagram including flow patterns of a
second optional retrofit of the four-stage bleaching plant depicted
in FIG. 7;
[0023] FIG. 10 is a schematic diagram including flow patterns of a
third optional retrofit of the four-stage bleaching plant with a
decommissioned Ep stage (tower and washer) depicted in FIG. 7.
[0024] FIG. 11 is a schematic diagram including flow patterns of a
fourth optional retrofit of the four-stage bleaching plant depicted
in FIG. 7.
[0025] FIG. 12 is a schematic diagram including flow patterns of a
prior art three-stage bleaching plant.
[0026] FIG. 13 is a schematic diagram including flow patterns of
one embodiment of a retrofit of the three-stage bleaching plant
depicted in FIG. 12;
[0027] FIG. 14 is a schematic diagram including flow patterns of a
second embodiment of a retrofit of the three-stage bleaching plant
depicted in FIG. 12; and
[0028] FIG. 15 is a schematic diagram including flow patterns of a
third optional retrofit of the three-stage bleaching plant with a
decommissioned P stage (tower and washer) depicted in FIG. 12.
DETAILED DESCRIPTION OF THE INVENTION
[0029] Retention time and medium consistency are the two
synergistic drivers for the present D.sub.emc technology and its
enhanced delignification performance and other benefits. At the
outset, it is noted that neither "medium consistency" nor "extended
retention time", when employed separately (i.e., not in
combination) is effective in obtaining the benefits of the present
combination which comprises the D.sub.emc technology. "Medium
consistency" for present purposes is defined as a consistency of
the pulp of at least 10%, and preferably between about 12% and
about 15%, based on the weight of oven-dried pulp. "Extended
retention time" for present purposes refers to the time which the
pulp resides within the D.sub.emc stage and may range between about
60 and about 180 minutes. The retention time in the current
invention is longer than commercial medium consistency D.sub.o
stage retention time. Lesser retention times reduce delignification
efficiency in the present invention and longer retention times do
not enhance the desired results.
[0030] Delignification efficiency in a process for conversion of
cellulosic material into pulp useful in papermaking is measured by
the kappa or K/P number reduction across D.sub.oE.sub.op stages
while selectivity is manifested by viscosity loss/kappa reduction
or filtrate COD/kappa reduction. In the prior art, the D.sub.o
stage comprises delignification employing treatment of a pulp with
chlorine dioxide at low consistency and short retention time. The
key operations and/or operational parameters of a typical prior art
D.sub.o stage are set forth in Table I above. In D.sub.emc
delignification according to the present invention, aside from
physical and mechanical changes, there is relatively little, if
any, change in the operational parameters of the prior art D.sub.o
stage other than pulp consistency and retention time and quantity
of chlorine dioxide consumed. Specifically, in accordance with the
present invention, the pH and temperature employed in the D.sub.emc
stage of are unchanged from current prior art operation. In the
D.sub.emc technology, the D.sub.emc stage temperature is determined
by that of the incoming pulp from a preceding operation (last brown
stock or post-O.sub.2 washer) and there is no steam addition
needed. Ideally, the D.sub.emc stage would be expected to operate
at lower temperature than the current D.sub.o stage because of the
faster chlorine dioxide reaction with lignin at the medium
consistency of the present invention than that at low consistency
D.sub.o. To the contrary, the present inventor has found that
higher D.sub.emc temperature (e.g., 67.degree. C.) does not
adversely affect D.sub.emc delignification performance, in that the
chlorine dioxide residual from the D.sub.o stage has been found to
not be as critical as it is for the D1 or D2 bleaching stages.
[0031] On the other hand, the major and important change in
converting the delignification stage, D.sub.o, to a D.sub.emc stage
lies in the change of the consistency of the pulp being treated in
this stage from a low consistency, i.e., 3-4%, to 10-15%
consistency and increase in the retention time of the pulp within
the D.sub.emc stage from between about 25 and <60 minutes to
between 60 and about 180 minutes. As noted, the benefits derived
from this combination of medium consistency pulp and extended
retention time of the pulp within the stage at the medium
consistency are not obtainable by either of these operational
parameters taken alone. There is a requirement that both the medium
consistency and the extended retention time be employed
simultaneously to obtain the benefits found to be provided by the
present invention. Table II below shows certain of the benefits
obtained by the present invention with respect to the chlorine
dioxide usage, the E.sub.op pulp P# and the Hex-A removal
percentage when processing O.sub.2 hardwood (HW) pulp having an
initial kappa of 10.5; a brightness of 45.8%; and a viscosity of
35.6 cPs.
2TABLE II Effect of D.sub.O Stage Consistency & Retention Time
on D.sub.OE.sub.OP Performance D.sub.O Stage Designation D.sub.OLC
D.sub.EMC D.sub.O Retention 25 75 130 Time, min D.sub.O
Consistency, 4 12 % D.sub.O CLO.sub.2, % 1 0.6 0.8 0.9 0.6 0.8 0.9
E.sub.op pulp bright- 69 65.5 68.4 71.2 64.8 69.4 69.7 ness, %
E.sub.op pulp P# 5.1 4.6 3.3 2.8 4 3.1 2.7 Hex-A Removal, % 20 33
40 54 32 52 58 Combined Filtrate 34.99 28.53 27.98 29.98 32.33
31.57 31.09 COD, kg/t
[0032] It is noted that, on the average, the chlorine dioxide usage
employed in the D.sub.emc stage is about 20% less than the prior
art D.sub.o stage This is contrary to the expectation of the prior
art wherein it is generally understood and practiced that chlorine
dioxide reacts with phenolic lignin within the first ten minutes
that the chlorine dioxide is in contact with the pulp, thereby
leading one skilled in the art away from the use of extended
retention time for less chlorine dioxide in an effort to reduce the
kappa or P# of the pulp, and especially away from the present
invention wherein not only is there less chlorine dioxide employed,
but there also is a reduction in the kappa or P# of the pulp.
Additionally, the present inventor has found that the benefits
afforded by the D.sub.emc technology are not a function of the type
(e.g., hardwood or softwood) pulp nor a function of the starting
kappa number or lignin content of the unbleached pulp.
[0033] P#, also known as permanganate number is a similar test to
kappa, indirectly measuring the lignin content of pulp. Many mills
use K number which is similar to P number. Generally, 1 P# or K# is
about 1.45-1.5 time kappa #.
[0034] Still further, it is noted that medium consistency, coupled
with the extended retention time in the D.sub.emc stage, enhances
the removal of Hex-A from the pulp. On the average, the present
invention removes 2 to -5 times as much Hex-A as does the prior art
D.sub.o process. In fact, a study on maple hardwood shows that the
D.sub.emc stage is capable of 70-80% Hex-A removal at a typical
chlorine dioxide charge for the current D.sub.emc stage, compared
with 60% for the prior art hot acid (A) stage employed for Hex-A
removal.
[0035] Selectivity can be loosely defined as the ratio of attack on
lignin to attack on carbohydrate. Practically, it measures the
degree of delignification (kappa reduction) over pulp viscosity or
pulp yield loss substituted sometime by filtrate COD-delta
kappa/delta viscosity or delta kappa/delta yield or delta
kappa/filtrate COD.
[0036] The present D.sub.emc technology has been employed in the
processing of different varieties of pulp. In addition to the
results shown in Table II these tests are set forth in Tables III
through VII below and have consistently borne out the benefits of
the D.sub.emc technology over the prior art D.sub.o technology.
3TABLE III Effect Of D.sub.O Stage Consistency & Retention Time
on D.sub.OE.sub.OP Performance Key D.sub.O Condition Difference
D.sub.O Stage Designation D.sub.OLC D.sub.EMC D.sub.O Consistency,
% 3.5 11 D.sub.O retention time, min 45 120 Pulp Properties D.sub.O
brightness, % 43.7 43.7 E.sub.OP brightness, % 60.6 59.4 DEK # 5.3
3 D.sub.OE.sub.OP filtrate COD, kg/t 38.89 32.75 Unbleached
Hardwood Pulp: 24.4% brightness, 38.3 cPs viscosity, 15.3 kappa
Other bleaching conditions: 1.3% CIO2 in D.sub.O and 0.2%
H.sub.2O.sub.2 in E.sub.OP in both cases The D.sub.emc stage uses
0.5% less H.sub.2SO.sub.4 (0.5% vs. 1%) and 0.2% less NaOH (1.2%
vs. 1.4%) than D.sub.OLC
[0037] As seen in Table III the same results as set forth in Table
II were obtained for a second hardwood pulp. That is, the D.sub.emc
stage decreases the DEK number from 3 to 5.3. Surprisingly, the
greater delignification efficiency in the D.sub.emc stage is
accompanied by reduced filtrate COD, indicating that the D.sub.emc
stage results in less non-lignin pulp organic dissolution. This
lower COD of less pulp organic dissolution can be translated to
less pulp yield loss during ECF bleaching and reduced energy
(aeration) expenditure in the waste water treatment plant (WWTP).
Generally a 10 kg/t COD can be approximated to a 1% pulp yield
loss.
[0038] In the test results reported in Table III, the D.sub.emc
stage used 0.5% less H.sub.2SO.sub.4 (0.5% versus 1%) and 0.2% less
NaOH in the subsequent E.sub.op stage (1.2% versus 1.4%) than the
prior art Do stage.
[0039] Conventional wisdom teaches that the chlorine dioxide
bleaching (D.sub.1 and D.sub.2) stages should be conducted at
higher (medium) consistency and longer (120-180 min) retention time
to maximize brightness development and shives bleaching. Tests on
hardwood and softwood pulps by the present inventor failed to show
such difference in pulp brightness, dirt count, viscosity, and
filtrate COD between the a D.sub.1LC stage and a D.sub.1MC stage.
In fact slight improvements were shown for the D.sub.1LC bleaching
than D.sub.1MC bleaching in pulp viscosity and filtrate COD,
indicating the slightly better bleaching selectivity of the low
consistency bleaching than the medium consistency bleaching.
[0040] The temperature in the low consistency D.sub.1 bleaching
stage was increased in some cases to compensate for the slower
chlorine dioxide reaction rate at low consistency pulp than at
medium consistency pulp. As an accompaniment to the present
D.sub.emc technology, the actual temperature in the low consistency
D.sub.1 stage should be practically determined to make sure that
all but a trace amount of chlorine dioxide is consumed for good
brightness development and shives removal. In general, 170.degree.
F. is needed for the low consistency D.sub.1 stage to ensure all
chlorine dioxide is consumed within the available retention time
for brightness and dirt bleaching.
[0041] While all D.sub.1 bleaching tests were conducted at 5%
consistency, the D.sub.1 stage at lower consistency (e.g., 4%) is
not noted to affect the bleaching results. The consistency of the
pulp in the bleaching stages should be maintained at the highest
possible, subject to the constraint of the available equipment to
minimize the filtrate recirculation and steam demand. These results
are shown in Tables IV-VIII below.
4TABLE IV Effect Of Consistency and Retention Time on Cl0.sub.2
Bleaching Efficiency Key D.sub.1 Stage Condition Difference D.sub.1
Stage Designation D.sub.1LC D.sub.1MC D.sub.1 Consistency % 5 10
D.sub.1 Retention time, min 60 120 Pulp Properties D.sub.1
Brightness % 83.4 84.2 D.sub.1 Viscosity, cPs 26.1 26.6 D.sub.1
dirt, ppm 0 0 Filtrate COD, mg/l 315 642 Eop Hardwood Pulp: 64.7%
brightness, 29 cPs viscosity, 2.2 P# D1 Stage CLO.sub.2 charge:
0.5%
[0042]
5TABLE V Effect Of Consistency and Retention Time on Cl0.sub.2
Bleaching Efficiency Key D.sub.1 Stage Condition Difference D.sub.1
Stage Designation D.sub.1LC D.sub.1MC D.sub.1 Consistency % 5 10
D.sub.1 Retention time, min 60 120 Pulp Properties D.sub.1
Brightness % 81.5 81.3 D.sub.1 Viscosity, cPs 23 21.2 D.sub.1 dirt,
ppm 0.04 0.1 Filtrate COD, mg/l 335 684 Eop Hardwood Pulp: 60.8%
brightness, 24.1 cPs viscosity, 4.3 P# Other D.sub.1 conditions:
0.5% CL0.sub.2 and 0.1% NaOH
[0043]
6TABLE VI Effect Of Consistency and Retention Time on Cl0.sub.2
Bleaching Efficiency Key D1 Stage Condition Difference D.sub.1
Stage Designation D.sub.1LC D.sub.1MC D.sub.1 Consistency % 5 10
D.sub.1 Retention time, min 60 120 Pulp Properties D.sub.1
Brightness % 84.8 85.4 D.sub.1 Viscosity, cPs 21.4 19.9 D.sub.1
dirt, ppm 0 0.08 Filtrate COD, mg/l 227 598 Eop Hardwood Pulp:
60.8% brightness, 24.1 cPs viscosity, 4.3 P# Other D.sub.1
conditions: 1.5% CL0.sub.2 and 0.4% NaOH
[0044]
7TABLE VII Effect Of Consistency and Retention Time on Cl0.sub.2
Bleaching Efficiency Key D.sub.1 Stage Condition Difference D.sub.1
Stage Designation D.sub.1LC D.sub.1MC D.sub.1 Consistency % 5 10
D.sub.1 Retention time, min 60 120 Pulp Properties D.sub.1
Brightness % 84.3 85.3 D.sub.1 Viscosity, cPs 24.8 24.6 D.sub.1
dirt, ppm 0 0 Filtrate COD, mg/l 175 365 Eop Hardwood Pulp: 70%
brightness, 26 cPs viscosity, 2.6 P# Other D.sub.1 conditions: 0.5%
CL0.sub.2 and 0.1% NaOH
[0045]
8TABLE VIII Effect Of Consistency and Retention Time on Cl0.sub.2
Bleaching Efficiency Key D.sub.1 Stage Condition Difference D.sub.1
Stage Designation D.sub.1LC D.sub.1MC D.sub.1 Consistency % 5 10
D.sub.1 Retention time, min 60 120 Pulp Properties D.sub.1
Brightness % 78.8 78.5 D.sub.1 Viscosity, cPs -- -- D.sub.1 dirt,
ppm 0 0 Filtrate COD, mg/l 292 650 Eop Hardwood Pulp: 52.3%
brightness, 27.7 cPs viscosity, 3.7 P# Other D.sub.1 conditions:
0.95% CL0.sub.2 and 0.1% NaOH
[0046] The advantages of the present invention have been found
adequate to justify retrofitting of existing bleach plants to
permit use of the D.sub.emc delignification technology without
cuting short of the current bleach stages.
[0047] It is shown that a short retention time--low consistency
chlorine dioxide bleach (D.sub.1LC) stage achieves the same or
better pulp brightness, dirt count, viscosity, and filtrate COD as
those from the currently practiced long retention time--medium
consistency bleaching (D.sub.1MC) stage. Thus, the current inventor
has found that one can use the currently used D.sub.o tower for
bleaching in lieu of the current D.sub.1 or D.sub.2 tower. This
factor has been found to permit the practice the D.sub.emc
technology of the present invention with the existing ECF bleach
plant equipment by merely swapping (through piping change) the
current D.sub.o and D.sub.1 tower and their respective functions
(delignification and bleaching/dirt removal). In this example, the
temperature in the low consistency (D.sub.1LC) bleaching stage
(conducted in the formerly D.sub.o tower) needs to be adjusted
based on the available retention time (available for the prior
existing D.sub.o tower) to ensure all of the chlorine dioxide is
consumed for brightness and dirt bleaching.
[0048] The present invention is applicable to all ECF bleach plants
(e.g., DEopD, DEopDD, DEopDP, DEopDEpD). As noted the key to
improved delignification efficiency and selectivity are medium
consistency of the pulp and extended retention time during its
first stage treatment (delignification) in the bleaching sequence.
Basically, the "old" D.sub.o stage is made the D.sub.emc stage or
the D.sub.emc technology is implemented in a tower which previously
had been employed for a D.sub.1 or a D.sub.2 stage. In either a
five, four or three stage bleaching plant, either the D.sub.1 or
the D.sub.2 tower currently existing in a bleach plant may be used
as the D.sub.emc tower (i.e., the first stage) and the currently
existing D.sub.o tower may be used for low consistency (D.sub.1LC),
a function currently achieved in the D.sub.1 or D.sub.2 stage. As
needed, a pretube can be added in front of the D.sub.1LC tower to
provide extra retention time for balancing the throughflow of pulp
through the bleach plant. Thus, in a four or three stage bleaching
plant, there is no loss of a stage, hence there is assurance that
the ultimately bleached pulp will be equivalent to the bleached
pulp produced by the plant prior to the implementation of the
D.sub.emc technology.
[0049] In accordance with one aspect of the present invention, the
inventors have found at least three ways to achieve medium
consistency for the D.sub.emc operation when retrofitting in an
existing ECF mill. First, the current brown or post-O.sub.2 high
density (HD) tower low consistency pulp delivery system may be
modified as by piping changes, for example, or second, a spare
washer may be inserted ahead of the bleach plant as a prewasher, or
third, the current last post-O.sub.2 washer after the HD tower pulp
delivery system may be switched to a bleach plant prewasher.
[0050] In addition to achieving medium consistency of the pulp, a
prewasher achieves additional washing which contributes to
reduction of filtrate COD and the overall bleaching cost.
[0051] FIG. 1 depicts a typical flow chart for a prior art
bleaching plant. As seen in FIG. 1, wood chips are loaded into a
digester 10, along with a pulping liquor (white liquor). The
digester may be operated on a batch basis or a continuous basis.
Continuous digesters are predominant in the present pulp and paper
industry. In the depicted embodiment, the output (brown stock) from
the digester is fed to a washer 12 wherein the brown stock is
washed and further delignified by oxygen (O2). Spent pulping liquor
from the washer may be recycled to chemical recovery. The washed
pulp stream from the O.sub.2 washer is fed to a bleach plant 16
wherein the pulp is subjected to a plurality of treatment stages,
each designed to convert the pulp to a useable source of
papermaking fibers. A typical bleach plant embodying the present
invention is adapted to provide an initial delignification stage
within the bleach plant, such delignification stage being followed
by one, usually two, or more bleaching stages wherein the pulp is
treated to develop one or more desired properties of the pulp,
depending in a major part on the type of paper to be made from the
pulp.
[0052] As noted, the present invention is applicable to all ECF
bleach plants (e.g., DEopD, DEopDD, DEopDP, DEopDEpD). Moreover,
the present invention is applicable to both softwood and hardwood
pulps.
[0053] As noted, the inventor's basic concept for retrofitting
currently existing bleaching plants to the use of the D.sub.emc
technology is to convert one of the bleaching towers (D.sub.1 or
D.sub.2, preferably) to the D.sub.emc stage. Specifically, the
inventor has found that the introduction of the D.sub.emc
technology into a certain currently existing bleaching plants
provides benefits of a nature and degree which permits a given
bleaching operation to produce equal or improved bleached pulp
while employing one less bleaching stage. This concept precludes
the need for additional major equipment expenditure, precludes
disruption of the overall balance of flows within the bleaching
plant, and requires relatively little expenditure for modifications
of the existing mechanical equipment. Primarily the changes involve
rerouting piping and as needed the addition of a pretube or the
like to provide additional retention time. Should spare or unused
equipment, such as a washer, be available in a given plant, this
washer may be included in the modified system as desired, again
without material capital or modification costs.
[0054] Alternatively, retrofit for the use of D.sub.emc technology
in the existing ECF bleach plants can be done by converting the
current low consistency D.sub.o stage to the D.sub.emc stage or by
swapping the function of the current D.sub.o and D.sub.1/D.sub.2
stages in which the current D.sub.1/D.sub.2 stage will be used for
the D.sub.emc stage and the current D.sub.o stage for the
D.sub.1/D.sub.2 stage without cutting short of the bleach
stages.
[0055] FIG. 2 schematically depicts a simplified version of a prior
art five-stage bleaching plant wherein digested and washed pulp is
fed into a D.sub.o stage, thence through E.sub.op, D.sub.1,
E.sub.p, and D.sub.2 stages, thence to a high density (HD) storage
tower.
[0056] FIG. 3 schematically depicts a first embodiment of a
modification to the bleaching plant depicted in FIG. 2 wherein the
current D.sub.o stage is converted to the D.sub.emc stage with the
remaining bleaching plant unchanged.
[0057] FIG. 4 depicts a second embodiment of a modification to the
bleaching plant depicted in FIG. 2 wherein the functions of the
current D.sub.o and D.sub.1 stages are swapped. The D.sub.1 stage
is converted to a D.sub.emc stage, and the D.sub.o stage is left in
place, but converted to a D.sub.1LC stage. In this embodiment, the
pulp initially enters the D.sub.emc stage, flows through the
E.sub.op stage, through the D.sub.1LC stage, through the E.sub.P
stage, through the D.sub.2 stage and is stored.
[0058] FIG. 5 depicts a third embodiment of a modification to the
bleaching plant depicted in FIG. 2 within the D.sub.o stage is
eliminated, the E.sub.p stage is converted to a P stage, the
D.sub.2 stage is converted to a D.sub.emc stage, and the output
from the D.sub.EMC stage is piped to the E.sub.op stage. Basically
this conversion involves little more than piping modifications.
[0059] FIG. 6 depicts a fourth embodiment of a modification to the
bleaching plant depicted in FIG. 2 within both D.sub.o and Ep
stages are eliminated, the D.sub.2 stage is converted to a
D.sub.emc stage, and the output from the D.sub.emc stage is piped
to the E.sub.op stage. Basically this conversion involves little
more than piping modifications.
[0060] FIG. 7 schematically depicts a simplified version of a prior
art four-stage bleaching plant wherein digested and washed pulp is
fed into a D.sub.o stage, thence through E.sub.op, D.sub.1, and
D.sub.2 stages, thence to HD storage.
[0061] FIG. 8 schematically depicts a first embodiment of a
modification to the bleaching plant depicted in FIG. 7 wherein the
current Do stage is converted to the D.sub.emc stage with the
remaining bleaching plant unchanged.
[0062] FIG. 9 depicts a second embodiment of a modification to the
bleaching plant depicted in FIG. 7 within the functions of the
current D.sub.o and D.sub.1 stages are swapped. The D.sub.1 stage
is converted to a D.sub.emc stage, and the D.sub.o stage is left in
place, but converted to a D.sub.1LC stage. In this embodiment, the
pulp initially enters the D.sub.emc stage, flows through the
E.sub.op stage, through the D.sub.1LC stage, through the D.sub.2
stage and is stored.
[0063] FIG. 10 schematically depicts a third embodiment of a
modification to the bleaching plant depicted in FIG. 7 wherein the
D.sub.o stage is eliminated, the D.sub.2 stage is converted to a
D.sub.emc stage, and the output from the D.sub.emc stage is piped
to the E.sub.op stage, thence it flows through the D.sub.1 stage,
to restored old Ep (converted to P) stage before to the HD
storage.
[0064] FIG. 11 schematically depicts a fourth embodiment of a
modification to the bleaching plant depicted in FIG. 7 wherein the
D.sub.o stage is eliminated, the D.sub.2 stage is converted to a
D.sub.emc stage, and the output from the D.sub.emc stage is piped
to the E.sub.op stage, thence it flows through the D.sub.1 stage,
to the HD storage.
[0065] FIG. 12 schematically depicts a simplified version of a
prior art three-stage bleaching plant wherein digested and washed
pulp is fed into a D.sub.o stage, thence through E.sub.op, and
D.sub.1 stages, thence to the HD storage.
[0066] FIG. 13 schematically depicts a first embodiment of a
modification to the bleaching plant depicted in FIG. 12 wherein the
current Do stage is converted to the D.sub.emc stage with the
remaining bleaching plant unchanged.
[0067] FIG. 14 depicts a second embodiment of a modification to the
bleaching plant depicted in FIG. 12 within the functions of the
current Do and D.sub.1 stages are swapped. The D.sub.1 stage is
converted to a D.sub.emc stage, and the D.sub.o stage is left in
place, but converted to a D.sub.1LC stage. In this embodiment, the
pulp initially enters the D.sub.emc stage, flows through the
E.sub.op stage, through the D.sub.1LC stage and is stored.
[0068] FIG. 15 depicts a third embodiment of a modification to the
bleaching plant depicted in FIG. 12 within the functions of the
current D.sub.o and D.sub.1 stages are swapped. The D.sub.1 stage
is converted to a D.sub.emc stage, and the D.sub.o stage is left in
place, but converted to a D.sub.1LC stage. The old decommissioned
Ep tower is converted to the P stage.
[0069] Table IX lists various permissible retrofits of current
bleaching sequences.
9TABLE IX Current Sequence Potential DEMC Sequence Potential
Bleaching Operations DE.sub.OPDE.sub.PD WD.sub.EMCE.sub.OPDP
Current D.sub.2 Washer as prewasher (W) Current D.sub.2 tower for
D.sub.EMC Current D.sub.O washer for D.sub.EMC washer Current
E.sub.P to P stage Spare current D.sub.O tower
D.sub.EMCE.sub.OPDE.sub.PD Current D.sub.O to D.sub.EMC stage
D.sub.EMCE.sub.OPD.sub.LCE.sub.pD Swap the current D.sub.O and
D.sub.1 stages WD.sub.EMCE.sub.OPD 1.sup.st case plus spare current
E.sub.P tower and washer DE.sub.OPDD WD.sub.EMCE.sub.OPD Current
D.sub.2 washer as prewasher (W) Current D.sub.2 tower for D.sub.EMC
Current D.sub.O washer for D.sub.EMC washer Spare current D.sub.O
tower D.sub.EMCE.sub.OPDD Current D.sub.O to D.sub.EMC stage
WD.sub.EMCE.sub.OPDP (for the mills Current D.sub.2 washer as
prewasher (W) with spare old E.sub.P stage) Current D.sub.2 tower
for D.sub.EMC Current D.sub.O washer for D.sub.EMC washer Spare
E.sub.P (if the mill has) to P stage Spare current D.sub.O tower
D.sub.EMCE.sub.OPD.sub.LCD Swap the current D.sub.O and D.sub.1
stages DE.sub.OPD D.sub.EMCE.sub.OPD Current D.sub.O to D.sub.EMC
stage D.sub.EMCE.sub.OPD.sub.LC Swap the current D.sub.O and
D.sub.1 stages D.sub.EMCE.sub.OPD.sub.LCP Swap the current D.sub.O
and D.sub.1 stages Spare E.sub.P (if the mill has) to P stage Spare
E.sub.P (if the mill has) to P stage
[0070] Table X shows the results of the stage-by-stage bleaching
performance, the current bleaching as a benchmark in the left
column of Table X and D.sub.EMC sequence on the right. In this
simulated modification of a typical four stage bleach plant
employing a D.sub.oE.sub.opD.sub.1D.sub.2 bleaching sequence, the
D.sub.o stage was converted to a D.sub.EMC stage with all other
stages remaining the same. Two additional simulated modifications
of the prior art D.sub.oE.sub.opD.sub.1D.sub.2 bleach sequence were
performed for WD.sub.EMCE.sub.OPD.sub.1LCD.sub.2 and
WD.sub.EMCE.sub.OPDP sequences, where "W" represents a spare washer
present within the plant. The results are summarized in Table X. As
expected, placing a prewasher stage before the D.sub.emc
delignification stage improves overall brightness development in
each of the two conversions. In the second of these two
modifications, a significant improvement in final pulp brightness
stability was achieved due to placing the P stage at the end of the
sequence. Additional benefits of these two modification were higher
pulp viscosity and lower filtrate emissions in the form of COD.
10TABLE X Bleaching Performance Summary Retrofit Case Control Case
1 Case 2 Case 3 Bleach Sequence D.sub.OLCE.sub.OPDD
D.sub.OLCE.sub.OPDD D.sub.EMCE.sub.opD.sub.LCD
WD.sub.EMCE.sub.opD.sub.LCD WD.sub.EMCE.sub.OPD.sub.LCD CLO.sub.2,
% 2.95 2.95 2.05 2.05 1.85 H.sub.2O.sub.2, % 0.2 0.2 0.5 0.5 0.7
NaOH, % 1.8 1.8 1.4 1.3 1.65 H.sub.2SO.sub.4, % 1 1 0.5 0.5 0.5
MgSO.sub.4, % 0 0 0.1 0.1 0.1 Pulp & Filtrate Properties
Brightness, % 87.1 87.3 86.7 87.7 87.5 Rev Brightness, % 85 85.1
84.5 85.6 86 Viscosity, cPs 20.6 20.9 23.5 24.4 22.6 Tappi Dirt,
ppm 0 0 0 0 0 COD, kg/t 43.87 43.8 37.64 39.02 42.69 Net Benefits
Cost Reduct., $/t Control Control 6.18 6.31 6.13 COD Reduct., kg/t
Control Control 6.23 4.78 0.11 Bleach Yield, % Control Control 0.6
0.45 0 Cost Basis: CL0.sub.2-$0.35/#, H.sub.20.sub.2-$0.2/#,
NaOH-$0.13/#, H.sub.2SO.sub.4-$0.04/#, MgSO.sub.4-$0.18/#
[0071] The retrofit costs for the D.sub.emc delignification as
shown in Case 1 (D.sub.EMCE.sub.OPD.sub.LCD) of Table X are, 1)
Modifications of current brown HD pulp delivery from low
consistency to medium consistency ready for the D.sub.emc stage
operation and the 2) The repiping relating to swapping the D.sub.o
with the D.sub.1 stage.
[0072] Upgrading the current spare E.sub.2 washer for a bleach
plant prewasher is part of the retrofit strategy in both Cases 2
and 3. Repiping was needed to reconfigure the bleach plant to
include the added changes.
[0073] The retrofitting strategy for Case 2
(WD.sub.EMCE.sub.OPD.sub.LCD) includes upgrading the spare E.sub.2
washer for a bleach plant prewasher to obtain medium consistency
for the D.sub.emc stage operation in addition to bleach stage
repiping to accomplish the exchanging of the functions of the
current D.sub.o and D.sub.1 stages.
[0074] Similarly, Case 3 (WD.sub.EMCE.sub.OPDP) requires upgrading
the current spare E.sub.2 washer for a bleach plant prewasher and
the E.sub.2 tower for the P stage. Either a current D.sub.1 or
D.sub.2 stage can be used for the new D.sub.emc stage
operation.
[0075] As the results in Table X show, the improved delignification
efficiency using the D.sub.emc technology contributed to a
significant amount of chlorine dioxide savings (19 lb/ton) along
with reductions in NaOH and H.sub.2SO.sub.4 usage relating to the
increased D.sub.o stage consistency. The peroxide usage in the
D.sub.emc technology is adjusted to be consistent with the general
industry trend in H.sub.2O.sub.2 usage in ECF bleaching. An average
peroxide usage in the E.sub.op stage of the modern ECF bleach
plants is 0.7-0.8%. The ECF bleach plants with O.sub.2
delignification operation usually use less peroxide in the E.sub.op
stage because of less brightness development at higher
H.sub.2O.sub.2 usage due to pulp's pre-exposure to similar
chemistry in the O.sub.2 stage.
[0076] Suboptimal peroxide usage in a current ECF bleach plant is
due to the desire to maximize pulp viscosity preservation. A 2
lb/ton MgSO.sub.4 addition was therefore accompanied with increased
peroxide use in the D.sub.emc ECF bleaching sequence. The net
results of a 6 lb H.sub.2O.sub.2 increase and 2 lb M.sub.gSO.sub.4
addition are 8% points (from 60% to 68%) higher E.sub.op pulp
brightness and almost 3 points higher bleached pulp viscosity, as
seen from Table X.
[0077] The filtrate COD is significantly decreased after
retrofitting conversion of the prior art four-stage bleach sequence
to the D.sub.emc sequence shown in Table X. Since filtrate COD
reflects the amount of organic dissolution from pulp during
bleaching, the reduced filtrate COD manifests the improved
bleaching selectivity of the D.sub.EMD technology over the current
four-stage bleaching sequence and represents the reduction in pulp
yield loss during the bleaching operation.
[0078] The overall results of the simulated modifications discussed
above are consistent with actual mill experience. Notably, the
results show about $6/ton or $1.5 MM/year bleaching cost reduction,
plus enhanced pulp brightness, reverted brightness, pulp dirt count
and viscosity attributable to the implementation of the D.sub.emc
technology. The improved bleaching selectivity is translated to
about 5 kg/ton COD reduction, representing about 0.5% pulp yield
increase and potential decrease in energy m the WWTP. All these
results are consistent with the demonstrated improved
delignification efficiency and selectivity of the D.sub.emc
technology over the prior art bleaching sequences from full-scale
operations of other mills and lab studies of pulps.
[0079] Mills which use high hexenouronic acid (Hex-A) content
hardwood species such as eucalyptus, maple, and birch, suffer from
low delignification efficiency (measured traditionally by the pulp
kappa difference before and after the O.sub.2 delignification
stage). This is due primarily to the inability of O.sub.2 to remove
part of the kappa which is attributable to the pulp Hex-A content.
For example, a maple pulp has a high Hex-A content and only about
30% delignification is achieved by a conventional oxygen stage.
[0080] High Hex-A content of a pulp not only interferes with the
delignification efficiency in the oxygen stage, but also inhibits
the delignification efficiency in the first two stages
(D.sub.oE.sub.op) of a prior art ECF bleaching plant. In one
example, the first two stages of a bleach plant may achieve only
about 40% delignification efficiency at kappa factor of about 0.26
in the D.sub.o stage with as much as 13 cPs unit viscosity loss.
This is due largely to the inability of the currently employed low
consistency, short retention time D.sub.o stage of the prior art to
effectively and selectively remove Hex-A.
[0081] Hex-A plays key roles in pulp bleaching chemical
consumption, AOX and COD formation, oxalate-related scale, and pulp
brightness stability particularly for hardwood species.
[0082] Hex-A reacts with permanganate and therefore contributes to
a significant amount of kappa number of unbleached pulps from a
digester or after oxygen delignification in a manner similar to
lignin. Hex-A is reactive towards electrophilic bleaching chemicals
such as chlorine dioxide in the D.sub.o, D.sub.1 or D.sub.2 stages
(more precisely Cl.sub.2 and HOCl generated during ClO.sub.2
reactions with lignin) and ozone in the Z stage, but stable toward
nucleophilic chemicals such as O.sub.2 in the O.sub.2 stage and
H.sub.2O.sub.2 in the E.sub.op and E.sub.p stages. Therefore, Hex-A
behaves similarly to lignin of the unbleached pulp and contributes
to chlorine dioxide consumption in the D.sub.o stage. Oxidation of
Hex-A increases the concentration of oxalate in filtrate, which can
potentially lead to increased scale in bleach plant equipment.
Hex-A contains unsaturated groups responsible for bleached pulp
brightness reversion, if not removed.
[0083] The Hex-A content in pulp depends on wood species and
cooking conditions. Hardwood (HW) pulps contain much higher Hex-A
than softwood (SW) pulps due to higher native xylan content in HW
than SW species and milder cooking conditions, i.e., lower alkali
concentrations, for HW than for SW. SW cooking typically
experiences a fast rise in Hex-A during the heating up period
followed by a gradual degradation of Hex-A in the subsequent
delignification period. In contrast, Hex-A content continues to
rise throughout the HW cooking period. The rate of Hex-A
degradation increases with increasing OH-, HS-, and ionic strength
of cooking liquor as well as increasing temperature. Higher alkali
residual with the modern modified cooking techniques drops the
Hex-A content in pulp.
[0084] Eucalyptus, maple and birch are known to contain high Hex-A
content among the HW species. For example, the contributions of
Hex-A to chlorine dioxide consumption and filtrate properties
during ECF bleaching of O.sub.2 delignified eucalyptus kraft pulp,
is reported in the literature. The contributions from residual
lignin are also listed for reference. This data is given in Table
XI.
11TABLE XI Characterization of Hex-A contributions of Eucalyptus
Pulp Total CIO2 Consump- OX, AOX, COD, Oxa- Kappa tion g/ton g/ton
kg/ton late Hex-A 5.19 55.2 197 7.57 % Contribution 62 42.4 35.5
37.9 40 35.5 Residual Lignin 3.1 62.7 37.4 8.49 % Contribution 48
41.61 40.4 12.2 45 30
[0085] Separate studies show that Hex-A in maple species accounts
for about 3K# (4.4 kappa) before O.sub.2 delignification and
2.8-2.9 K# after O.sub.2 delignification. These results are shown
in the following TABLE XII.
12TABLE XII Contribution of Hex-A to Pulp K# of Maple Pulp Sample
Hex-A ug/g Initial K# Final K# K# from Hex-A Pulp Before O2
Delignification (#2 BSW) #1 3756.8 9.8 6.9 2.9 #2 3889.0 10.2 6.5
3.7 Pulp After O.sub.2 Delignification (#1 P.sub.OW) #1 3000.7 7.7
4.9 2.8 #2 3550.4 7.6 4.7 2.9 *Hex-A removed
[0086] Hot acid (A) treatment is considered as an effective means
to selectively remove Hex-A (by about 60%) prior to chlorine
dioxide delignification, hence reduction of the chlorine dioxide
consumption by the Hex-A. Hot acid treatment, however, requires a
significant amount of retention time (usually 90-120 minutes), high
temperature (75.degree.-90.degree. C.), and an acidic pH (2.5-3.5).
These operating parameters require a large acid resistant tower,
hence a very large capital expenditure ($1.5-$3 million). Moreover,
the large amount of acid required for the reaction and the large
amount of steam required to develop and maintain the high
temperature add to the overall cost of an A stage.
[0087] The currently used D.sub.o stage of an ECF bleach plant
removes a very limited amount (10%-30%) of the Hex-A in a hardwood
pulp. This is at the expense of consumption of the chlorine
dioxide. The lower Hex-A removal efficiency in the prior art
D.sub.o stage (low pulp consistency and short retention time) has
been noted to be due to both lower efficiency (diffusion,
reactions, etc.) and insufficient retention time required for acid
hydrolysis.
[0088] In accordance with one aspect of the present invention
employing D.sub.emc technology in the initial delignification
stage, the present inventor discovered that there occurs
simultaneous enhanced delignification plus Hex-A removal, hence
reduction of the chlorine dioxide demand, as well as environmental
(filtrate volume, COD/AOX emission) and operation and quality
improvement, all at reduced cost.
[0089] TABLE XIII below illustrates the performance difference
between the prior art Do technology and the present D.sub.emc
technology from another mill study.
13TABLE XIII Lab Investigation of O.sub.2 Delignified Pulp Pulp #1
Pulp #2 Sequence D.sub.LCE.sub.OP D.sub.EMCE.sub.OP
D.sub.LCE.sub.OP AD.sub.EMCE.sub.OP E.sub.OP Brightness, % 78.7
85.3 73.7 81.9 E.sub.OP P# 3.5 1.2 5.1 2 E.sub.OP Viscosity 19.2
21.8 19.1 28.4 Filtrate COD, kg/t 50.05 44.6 Where A-is acid
treatment with H.sub.2SO.sub.4 Pulp #1- 52.6% brightness, 27.7% cPs
viscosity, and 6.3 P# Pulp #2- 41.2% brightness, 32.9% cPs
viscosity, and 10.8 kappa or 7.4 P#
[0090] Laboratory studies by the present inventor on HW pulps for
an operating mill (collected at two different times with different
unbleached pulp kappa or P# and brightness) consistently showed
that the prior art D.sub.oLC stage is not only inefficient (about
33% and 44% P# reduction in the D.sub.oE.sub.op stages), but also
very non-selective (at the expense of 8.5 and 13.8 viscosity drop
in these stages). In comparison, the present D.sub.emc stage
significantly increases P# reduction efficiency in the first two
stages over the prior art D.sub.oLC stage to 80-81% while reducing
viscosity-loss to about 5 cPs. The improved delignification
selectivity of the D.sub.emc stage is also manifested by 10%
reduction in filtrate COD, which can be translated to a higher pulp
yield over the prior art D.sub.oLC stage. It is noted that P# and
previously used K# are similar terms used to indicate the lignin
and Hex-A content in the pulp.
[0091] The low P# reduction efficiency of the prior art
D.sub.oLCE.sub.op stages (shown in Table XIII) is due to the high
content of Hex-A in HW pulps (esp. maple) and the relative
inability of the prior art to remove the Hex-A. Assuming a
delignification efficiency of 70% in the prior art
D.sub.oLCE.sub.op stages (normally, DoLcEop stages achieve 70-80%
delignification at a kappa factor of 0.2=0.3 in the D.sub.o stage),
the D.sub.oLCE.sub.op stages remove only about a calculated 13% of
the Hex-A in the HW pulp as shown in TABLE XIV below.
14TABLE XIV Lignin and Hex-A Removal Efficiency in the
D.sub.LCE.sub.OP Stages on Pulp #1 Lignin Hex-A Total Unbleached
Pulp P# 3.5 2.8 6.3 D.sub.LCE.sub.OP Pulp P# 1.05 2.45 3.5 Removal
Efficiency, % 70 13 44
[0092] Further, assuming 80% delignification by the
D.sub.emcE.sub.op stages (10% higher than that of the
D.sub.oLCE.sub.op stages), it is derived that the D.sub.emcE.sub.op
stages simultaneously remove 82% Hex-A from the pulp. This is shown
in TABLE XV below.
15TABLE XV Lignin and Hex-A Removal Efficiency in the
D.sub.EMCE.sub.OP Stages on Pulp #1 Lignin Hex-A Total Unbleached
Pulp P# 3.5 2.8 6.3 D.sub.EMCE.sub.OP Pulp P# 0.7 0.5 1.2 Removal
Efficiency, % 80 82 81
[0093] Since E.sub.op is incapable of significantly removing Hex-A
from the pulp as is possible in an O.sub.2 stage, it is observed
that it is the chlorine dioxide that removes the majority of the
Hex-A in the pulp at the conditions present in the D.sub.emc
stage.
[0094] TABLE XVI below is a performance comparison between
D.sub.emcE.sub.op stages and AD.sub.emcE.sub.op stages and shows
that addition of a hot acid (A) treatment on maple pulp #2 does not
achieve additional P# or Hex-A reduction efficiency improvement
over the D.sub.emcE.sub.op stages treating pulp # 1, suggesting
that the function of the D.sub.emc and A stages are overlapping and
redundant in terms of their ability to remove Hex-A. In fact,
addition of an A stage to the D.sub.emcE.sub.op stages resulted in
an overall lower delignification and Hex-A removal efficiency (by
comparing the results of the two pulps in Table XVI) implying
potential interference of the A stage with the D.sub.emc stage
performance. The potential interference of the A stage with the
D.sub.emc stage delignification efficiency leads to a conclusion on
the potential synergistic effect of Hex-A oxidation products, e.g.
oxalate, on lignin removal (delignification) efficiency in the
D.sub.emc stage.
16TABLE XVI Performance Comparison Between D.sub.EMCE.sub.OP and A
D.sub.EMCE.sub.OP Stages Pulp #1 w/ D.sub.EMCE.sub.OP Pulp #2 w/ A
D.sub.EMCE.sub.OP Lignin Hex-A Total Lignin Hex-A Total Unbleached
3.5 2.8 6.3 4.6 2.8 7.4 Pulp P# D.sub.EMCE.sub.OP 0.7 0.5 1.2 1.38
0.62 2 Pulp P# Removal Effi- 80 82 81 70 78 80 ciency, %
[0095] Further, for mills which are currently processing high Hex-A
pulps species (e.g., maple) and currently employing a D.sub.oLC
stage, it is more cost-effective to invest in D.sub.emc technology
to enhance both Hex-A and lignin removal efficiency than to invest
in the addition of an A stage to achieve Hex-A removal only.
[0096] The advantages of the D.sub.emc technology over the prior
art D.sub.o (which is carried out with low consistency pulp)
technology indicates the importance of higher concentration (as a
result of higher pulp consistency) and longer retention time in the
removing of Hex-A from the pulp, along with the other noted
advantages of the D.sub.emc technology. As has been demonstrated by
the present inventor, the improved efficiency and selectivity of
the D.sub.emc technology emanates from (a) increased consistency of
the pulp (higher chemical concentrations leading to faster reaction
kinetics and mass transfer as well as longer retention time) and
(b) extended retention time which benefits only at medium
consistency of the pulp.
[0097] Given the complex reactions occurring simultaneously in the
initial delignification stage of digested pulp treatment (either
D.sub.o or D.sub.emc stage)--some being productive and some being
unproductive--increased pulp consistency and chemical concentration
can alter the ratio of productive to unproductive chemical
reactions in this initial stage toward ultimately positively
affecting delignification efficiency and selectivity. For example,
from a reaction mechanism point of view, high consistency and
chemical concentrations favor formation of ClO.sub.2 over
ClO.sub.3--, minimizing wasting oxidizing power of ClO.sub.2. It is
also postulated that increasing the concentration and duration of
more effective delignification chemicals such as ClO.sub.2 and HOCl
in the D.sub.emc stage as a result of increased pulp consistency,
leads to overall enhanced delignification efficiency. The enhanced
efficiency and selectivity may be by increasing the concentration
of efficiency-enhancing components (produced in the D.sub.emc
stage) to a critical level as a result of the increase in pulp
consistency that is not possible in the low consistency D.sub.o
stage. One example may be increased oxalic acid or oxalate
concentration formed in oxidation of lignin and Hex-A in the
D.sub.emc stage.
[0098] Again, recognizing the complexity of the chemical reactions
which take place in an initial chlorine dioxide-based
delignification stage of digested pulp, and whereas the present
invention is not intended to be limited to any particular chemical
reaction or combination of chemical reactions within an initial
chlorine dioxide-based delignification stage. It is believed that
medium consistency pulp leads to high chemical concentrations at a
given reactor volume and chemical charge and a fast delignification
rate. The higher the consistency, the faster the delignification
kinetics. High chemical concentration maintains high driving forces
and improves overall reaction efficiency and selectivity by
minimizing secondary reactions. Like O.sub.3, the ClO.sub.2
delignification process is generally considered to be diffusion
controlled because of the low delignification activated energy
(52-64 kj/mol). High pulp consistency aids the diffusion process by
maintaining high ClO.sub.2 concentration and decreasing the
thickness of the diffusion layers. Increasing Do consistency from
the prior art 3-3.5% to 10% or higher increases the D.sub.o stage
retention time by 3 times the prior art time, e.g. from 30 min. to
90 min. From a reaction mechanism point of view, high consistency
pulp and chemical concentration favors formation of ClO.sub.2 over
ClO.sub.3, minimizing wasting oxidizing power.
[0099] With respect to the effect of retention time of the pulp
within the reactor vessel, ClO.sub.2 reacts wit lignin
(particularly nonphenolic lignin) much slower than elemental
chlorine and similar to oxygen, chlorine dioxide delignification is
typically represented by two-phase reaction kinetics. The long
retention time improves chlorine dioxide delignification and
subsequent extraction efficiency by completing secondary slow
delignification reactions with nonphenolic lignin in the chlorine
dioxide delignification stage of the pulp.
[0100] In any event, the benefits of both retention time and
consistency of the pulp are evident in the D.sub.emc stage is shown
in the following TABLES XVII, XVIII, and XIX.
17TABLE XVII Synergistic Effect Of Retention Time And Medium
Consistency D.sub.O Brightness D.sub.O CLO.sub.2 Charge, % 0.4 0.75
1 4% CSC & 25 min in D.sub.O 70.6 72.2 73.6 12% CSC & 25
min in D.sub.O 72.1 75.6 76.7 12% CSC & 130 min in D.sub.O 72.1
76.4 78.2
[0101]
18TABLE XVIII Synergistic Effect Of Retention Time And Medium
Consistency On E.sub.OP Brightness D.sub.O CLO.sub.2 Charge, % 0.4
0.75 1 4% CSC & 25 min in D.sub.O 75.4 75.5 78.7 12% CSC &
25 min in D.sub.O 76.8 79.8 82 12% CSC & 130 min in D.sub.O
77.9 83.4 85.3
[0102]
19TABLE XIX Synergistic Effect Of Retention Time And Medium
Consistency on E.sub.OP P# D.sub.O CLO.sub.2 Charge, % 0.4 0.75 1
4% CSC & 25 min in D.sub.O 3.9 3.7 3.5 12% CSC & 25 min in
D.sub.O 3.4 3 2.7 12% CSC & 130 min in D.sub.O 3.4 2.3 1.2
[0103] However, the beneficial effect of retention time in the
D.sub.emc stage is absent in the prior art D.sub.oLC stage as shown
in TABLE XX below. In short, there is no effect on retention time
in the prior art D.sub.oLC stage.
20TABLE XX Synergistic Effect Of Retention Time In Low Consistency
(3.5%) D.sub.O Delignification Retention Time, min 30 90 180
Temperature, C. 60 58 55 CLO.sub.2, % 1.1 1.1 1.1 Do Brightness, %
51 51 50.2 E.sub.OP Brightness, % 72 72.4 71.3 E.sub.OP P# 2.5 2.5
2.4
[0104] The enhanced Hex-A removal and overall P# reduction
efficiency in the D.sub.emc stage over the prior art D.sub.oLC
stage could also imply (a) the potential synergistic effects of
Hex-A oxidation products, e.g., oxalate, on lignin removal
(delignification) efficiency in the D.sub.emc stage because of
their reaching critical concentrations which is not possible in the
prior art D.sub.oLC stage and/or (b) the extended acid hydrolysis
in the D.sub.emc stage which may contribute to the enhanced Hex-A
removal that is absent in the prior art D.sub.oLC stage.
Implication (a) points to the importance of higher consistency and
implication (b) points to extended retention time.
[0105] Whereas the present invention has been described at times in
specific terms and specific examples, it is intended that the
invention be limited only as set forth in the claims appended
hereto.
* * * * *