Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
7~
BLEACHING OF CELLULOSIC MATERIALS
BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates generally to improvements in processes
for bleaching cellulosic materials in the form of pulp, such as wood
pulp, and particularly to the rapid bleaching of pulps.
DESCRIPTION OF THE PRIOR ART
Pulping is the changing of wood chips or other lignocellulosic
material to fibrous form. Chemical pulping requires cooking of such
material in solution with a chemical in a " digester" . The process
includes partial removal of the coloring matter such as l;gnin asso-
ciated with the wood.
Pulp, as it comes from the digester, whether produced from
hard-wood or softwood or from non-woody materials, contains resid-
ual coloring matter. While unbleached pulp may be used for the
manufacture of certain grades of paper, for example, heavy
wrapping paper and paper for use in bags, pulp which is to be
used for printing or paper or sanitary paper must be bleached.
Depending upon the nature of the raw pulp and the end use of
which the pulp will be employed, various chemical bleaching stages
and various sequences of these stages have been used heretofore.
Among the principal chemical bleaching stages which have been used
are the chlorination stage (designated "C"), the caustic extraction
stage (designated "E"), the hypochlorite stage (designated "H"),
and the chlorine dioxide stage (designated "D"). In addition, both
chlorine and chlorine dioxide may be used in the same stage (desig-
nated IICD") or (''Dc'') and the chemicals may be used as a mixture
or added sequentially. Various combinations of the above stages
have been employed depending upon the specific conditions and
bleaching requirements. For example, common bleaching sequences
may include the following: CEH, CEHD, CEHED, CEHDED, and
CEDED. Of these, the DEDED and CDEHDED are the recognized
standard sequences for producing 88+GE Brightness market pulp.
' ~e
'~
The CDEDED sequence produces a high brightness pulp with a
minimum of vicosity loss or cellulose degradation to the pulp. This
results in a pulp which has high strength properties. The
CDEHDED sequence also produces high brightness, but heretofore
the hypochlorite stage has caused degradation of the cellulose albeit
in a controlled fashion. This results in some loss in paper
strength, but the pulp requires less mechanical beating in order to
develop its maximum strength, compared with pulp bleached by the
CDEDED sequence. The CDEHED sequence is used to make the
10 same type of pulp as the CDEHDED sequence, but it has one fewer
stage for control of brightness. The CEH sequence is used for
semibleached pulp in the brightness range 65 to 75 GE Brightness
Standard. The CDEHD sequence has not normally been used for
pulp requiring brightness greater than 86 C~E because bleaching to
15 higher brightness with this sequence generally results in a severe
loss in viscosity and strength.
In the initial chlorination stage, chlorine is added to the
washed pulp received from the digester. Ordinarily, the chlorina-
tion stage (C) is performed at temperatures in the range of about
20 30C to 50C, with a pulp consistency of about 3 percent. Under
these conditions, the reaction time in the chlorination tower is about
30 to 60 minutes. The chlorine reacts directly with the lignin and
other impurities in the pulp. Chlorine dioxide may be used in
conjunction with chlorine ( CD ) or in place of chlorine for the initial
25 chlorination stage (D).
Following the chlorination stage, a caustic extraction stage (E)
using a dilute aqueous solution of sodium hydroxide (0 . 5 to 5 . 0
percent NaOH based on oven-dry weight of pulp) is performed to
dissolve the chlorinated and oxidized lignin as well as some of the
30 resin. The extraction stage is usually performed at temperatures of
about 50C to 80C for a period of about 60 to 120 minutes with a
pulp consistency of 10 to 20 percent.
The next stage of bleaching is commonly a hypochlorite stage
(H), although a chlorine dioxide stage is sometimes preferred. In
35 the hypochlorite stage, either sodium hypochlorite (NaOCI) or
37~
-- 3 --
calcium hypochlorite (Ca(OCl)2) is used to further oxidize the
remaining lignin and other impurities in the pulp. Some degracla-
tion of the pulp as a result of shortening the chain length of the
cellulose molecule usually occurs in the hypochlorite stage.
5Following the hypochlorite stage, there may be a second alka-
line or caustic e~traction stage (E) or a chlorine dioxide stage (D).
SUMMARY OE` T~E INVENTION
The present invention provides an improved process for the
10delignification and bleaching of lignocellulosic material wherein the
material is treated with chlorine dioxide (D) in combination with
hypochlorite (H) in the presence of an acid.
DESCRIPTION OF THE PREFEE~REI) EMBODIMENTS
15Bleaching is the treatment of cellulosic fibers to remove or
alter the coloring matter associated with the fibers to allow the
fiber to reflect white light more truly.
The present invention is concerned with the degradation of
lignin and hence "delignification" and "bleaching" are used inter-
20-changeably.
Consistency is the weight of pulp fibers in a pulp suspension
usually expressed as a percentage. For e~ample, one pound oven
dry fiber in one hundred pounds of a mixture of water and fibers
would be a pulp suspension of lg~ consistency. The consistency of
25the pulp will depend upon the type of dewatering equipment used.
One measure of the efficacy of a bleaching process is the
degree of delignification. There are many methods of measuring the
degree of delignification of the pulp, but most are variations of the
permanganate test.
30The permanganate test used ~erein provides a Permanganate
number (KNO) a measure of potassium permanganate solution con-
sumed by o~ren dry pulp under specified conditions. The Perman-
ganate number is determined in accordance with TAPPI standard
test method T 214 SU-71.
,.
Brightness in the examples which follow was measured with an
Elrepho tTrade Mark) brightness meter in accordance with TAPPI
standard T217M. Viscosity measurements were made using a capillary
viscometer method as outlined in TAPPI procedure T-230. The
5 viscosity value ( centipoise or " CPS" ) is useful to determine the
degree of cellulose degradation during bleaching.
In the Tables, "oven dried" is abbreviated "OD",
"hypochlorite" is abbreviated "hypo" and "centipoise" is abbreviated
" cps " .
The principles, features and advantages of the invention will
be further understood upon consideration of the following specific
examples .
Example 1
An 85 gram sample of unbleached Northern softwood kraft pulp
15 having a Permanganate number of 17 . 5 was mixed with chlorine
water at a charge level of S . 6g6 chlorine based on oven dry weight
of the fibers in a closed glass bottle in a consistency of 3 . 596 . The
bottle was immersed in a water bath maintained at 29C for 45
minutes. After the retention, the chlorinated pulp was washed well
20 with tap water on a vacuum drain Buchner funnel employing filter
paper. The chlorinated pulp was then subjected to caustic extrac-
tion employing the following conditions: 2. 5% NaOH based on oven
dry pulp, 65.5C, 90 minutes and 11% consistency. The extracted
pulp was washed in the same manner described above for the chlori-
25 nated pulp. 5g of the chlorinated and caustic extracted pulp (CEpulp) was used for CEK number determination (the Permanganate
number after the CE stage) . The determined CEK number was 3 . 3
which was in the normal range practiced in a pulp mill (3.0-6.0).
The remaining 80 grams of the CE pulp were equally divided into
30 two 40 gram batches. The first batch was treated with chlorine
dioxide alone as a control c~f first chlorine dio~ide bleaching stage
(called D1 stage). The control Dl bleaching was carried out under
the following conditions: 1. 196 ClO2 charged based on OD pulp,
63C, 150 minute retention time and 1096 consistency. In this ClO2
35 bleaching experiment the CE pulp mat was broken up in a Hobart
~`:
~2~7~
mixer with addition of the required amount of dilution water for
consistency adjustment. The well broken-up pulp slurry was then
transferred into a glass bottle, equipped with air-tight cap. Prior
to addition of chlorine dioxide solution the pulp slurry in glass
5 bottle was preheated to 57-63C. The required amount of chlorine
dioxide solution was added to the pre-heated pulp slurry in the
bottle. Upon adding chlorine dioxide solution the bottle opening
was closed immediately with an air-tight cap and was shaken vigor-
ously to promote a thorough mixing between the chlorine dioxide
10 solution and the pulp slurry. After said mixing the glass bottle
was immersed in a water bath maintained at 63C. At the end of
the bleaching reaction, the pulp was checked for pH and residual
chlorine dioxide and then washed with tap water in the same manner
described before. A handsheet weighing 5 grams was prepared for
15 brightness measurement of the D1 stage. After air drying the
sheet, the initial brightness was measured. The reverted brightness
was read after aging the same sheet in a hot air circulating oven at
40 . 6C for 60 minutes.
The second 4Q grams of CE treated pulp mat was broken up in
20 a Hobart mixer. Thereafter, lOOml of carbonic acid, which was
prepared by saturating CO2 gas into deionized water, was added
while agitating. Immediately after the carbonic acid addition, a
sodium hypochlorite solution was added at a charge level of 0 . 55%
available chlorine based on oven dry weight of fiber with agitation.
25 Upon the hypochlorite addition, the carbonic acid and hypochlorite
mixed pulp slurry was transferred to a glass bottle equipped with
an air-tight cap and preheated to 57-63C. After the preheating, a
chlorine dioxide solution was added at 50% of the charge level in the
control bleaching, O . 559~ C102 based on ovendry weight of fiber in
30 the pulp. The tightly closed glass bottle containing pulp fibers,
carbonic acid, sodium hypochlorite and chlorine dioxide solution was
immersed in a water bath maintained at 63C for 90 minutes. The
pulp consistency in the bottle was 10g6, just like the control D]
stage bleaching. The bleached pulp was washed and measured for
35 brightness in the same manner of the first batch.
7~
After the first chlorine dioxide bleaching, the two pulp
samples were further subjected to the second caustic extraction
(E2) and second chlorine dioxide bleaching (D2) under the same
conditions as follows:
E2 stage: 0. 5% NaOH based on OD pulp
74C, 60 minutes retention time
1196 consistency
D2 stage: 0 . 4% ClO2 based on OD pulp
74C, 200 minutes and 10% consistency
After the D2 stage, initial and reverted pulp brightness and
viscosity of the fully bleached pulp were measured.
The first batch thus was bleached in multi-stage bleaching
15 sequence CED1E2D2 and the second batch was bleached in multi-
stage bleach sequence CE(H+D)E2D2. The chlorine dioxide bleach-
ing with a combination of acid and sodium hypochlorite solution is
referred to here as (H+D). The bleaching results of control CEDED
and CE(H+D)ED bleaching are summarized in Table 1.
TABI.E 1
Batch 1 Batch 2
Bleaching Sequence CED1E2D2 CE(H+D)E2D
D1 or (H+D) Stage
96 C102 on OD Pulp 1.1 0. 55
25 % Hypochlorite on OD Pulp 0 . 0 0 . 55
% Substitution of Hypochlorite
12 0.0 50 0
% Total Available Chlorine Used2 . 90 2 . 00
30 Dl Stage Brightness Initial 81.1 81. 2
Reverted77 . 2 76 . 5
D2 Stage Brightness Initial 88 . 6 88 . 4
Reverted84 . 8 84 .3
D2 Pulp Viscosity (cps) 14.1 14.6
7~
-- 7 --
From the results shown in Table I, it is evident that sodium
hypochlorite can be substituted for 50% of the ClO2 normally used
in the first chlorine dioxide stage in the multistage bleaching
seguence CEDED. The chlorine dioxide bleaching with a combination
of carbonic acid and hypochlorite gave the same brightness obtained
with 100% chlorine dioxide in the first chlorine dioxide bleaching
stage. Contrary to the teachings of the conventional chlorine
dioxide and hypochlorite bleaching art, the addition of acid and
hypochlorite at the high temperature did not cause the expected
10 viscosity drop during the (H+D) bleaching.
In this example, one part by weight of hypochlorite replaced
one part by weight of chlorine dioxide even though one part by
weight of hypochlorite is equivalent to 0 . 38 part by weight of
chlorine dioxide in terms of oxidi~ing equivalent. In this novel
bleaching process, one part by weight of hypochlorite reacts like
one part of chlorine dioxide in the presence of acid . The ( H+D )
bleaching therefore reduced the total available chlorine usage in the
first chlorine dioxide bleaching stage from 2 . 9% to 2.0%, which was
a 31% reduction.
Example 2
A 100 gram sample of unbleached Southern pine kraft pulp hav-
ing a Permanganate number of 18 . 5 was treated, as described in
Example I, with an aqueous solution of chlorine so that 5.94%
chlorine was added based on the weight of the fibers. The chlori-
nation treatment was carried out at a consistency of 2.8% for 30 minat 32C. The chlorinated pulp was washed in the manner described
in Example 1. The chlorinated pulp, at 3.5% consistency, was then
treated with sodium hypochlorite solution and caustic soda solution
for 20 minutes at 38C. The hypochlorite solution was added at a
charge level of 1. 596 available chlorine based on oven dry weight of
fiber, at the same time the caustic solution was added at a charge
level of 2.0% NaOH based on oven dry weight of fiber.
The chlorinated and hypochlorite treated (CH) pulp was
equally divided and one batch was bleached with chlorine dio~ide
alone as a control D1 bleaching and the other was bleached with a
combination of carbonic acid and sodium hypochlorite and chlorine
dioxide. The chemical charge level was 0 . 9% ClO2 based on oven
dry weight of fiber in the first batch and for the second batch
sodium hypochlorite solution was added at a charge level of 0 . 45%
which is 50% of the 0 . 9% ClO2 added in the first batch, along with
60 cc carbonic acid solution and 0 . 45% ClO2 . The adclition was
made in the same manner as in Example 1.
After the D1 bleaching for the first batch and the (H+D)
bleaching for the second batch, air-dried brightness was measured
10 for the two bleached samples. The two batches were then further
treated with second caustic extraction stage with addition of sodium
hypochlorite ( EH stage) and then with chlorine dioxide bleaching in
the second D stage under the same conditions. The bleaching
results on Southern pine kraft pulp are shown in Table II,
TABLE II
CHDlEHD2 CH(H+D)EHD2
Batch 1 Batch 2
D1 or (H+D) Stage
% ClO2 on OD Pulp 0, 9% 0 . 45
% Hypochlorite on ODPulp 0 . 0 0, 45
Volume of Carbonic Acid Added 0.0 60cc
D1 Stage - End pH 3.3* 2.6
Brightness 71. 2 71. 3
Total Available Cl2 Used (%) 2 . 37 1. 63
D2 Stage
Initial Brightness 87 . 7 87.8
Reverted Brightness 82 . 4 83 . O
Pulp V~scosity, cps 7.7 7.7
* 0 . 25% NaOH was added to the D1 stage as pH buffer.
EH conditions: 0.7% NaOH + 0.45% sodiuQ~ hypochlorite, 71C~ 60 Min.
D2 conditions: 0.4% ClO2 on OD pulp, 74C, 90 Min.
~ ` .
~%~
_ 9 _
As the result in Table II shows, it is again evident that 50% of
the chlorine dioxide used in the first dioxide bleaching stage can be
replaced by hypochlorite at an equal weight basis without affecting
the Dl stage brightness as well as the final brightness after the D2
5 stage in multistage bleaching process CHDlEHD2. It is again demon-
strated that addition of hypochlorite and acid did not cause any
pulp viscosity drop. Example 2 also proves that the (H+D) bleach-
ing is applicable whether the chlorinated pulp is treated with
caustic extraction or with alkaline hypochlorite. It should be noted
10 that the total available chlorine usage in the first chlorine dioxide
bleaching stage was reduced by 31g6 in the (H+D) bleaching as com-
pared to the control Dl bleaching.
Example 3
The unbleached pulp employed in this example was Canadian
softwood kraft pulp obtained from a mill located on the east coast of
Canada. The Permanganate nuMber of the unbleached pulp was 16.3.
After treating the pulp by chlorination with 4.63% C12 at 2 . 8%
consistency for 35 minutes at 38C, the chlorinated pulp was then
subjected to alkali/oxygen ~E/o) treatment in a modified laboratory
20 pressurized reactor under the following conditions:
% NaOH : 2 . 315% on OD weight of fiber
Consistency : 15%
Retention Time : 45 minutes
Temperature : 74C
2 Pressure : 100 PSIG (7 Kg/cm2)
p~ InlOut : 12.4/10.5
Permanganate number
after CE I O : 2, 5
80 grams of the oxygen treated pulp sample was equally divid-
30 edinto two batches; one was treated with 0. 9% C102 (based orl OD
pulp ) and the other was treated with 0 . 45% ClO2 plus 0 . 45% sodium
hypochlorite with addition of 50cc of carbonic acid~ After the Dl
and the (H~D) bleaching, the pulp was further treated with caustic
extraction and chlorine dioxide bleaching under the same conditions.
,,
~,
-- 10 --
The results of bleaching sequences CE/ODED and CE/O(H+D)ED are
shown in Table III.
TABLE III
Sequence CE/ODlE2D2 CElo(H+D)E2D2
D1 or (H+D) Stage
% ClO2 on OD Pulp 0 . 9 0, 45
% Hypo on OD Pulp 0 . 0 0 . 45
Temperature (C) 60 60
Time (Min. ) 120 110
Consistency (%) 8 8
15 D1 Brightness
Initial 77 . 6 78 . 6
Total Available Chlorine(% Cl2) 2.37 1.63
20 D2 Stage
Initial Brightness 87 . 2 87 . 8
Reverted Brightness 83 . 7 83 . 7
E2 stage conditions : 0 . 35% NaOH on OD pulp , 71C , 120 Min ., 12%
consistency
D2 stage conditions: 0. 2% Cl02 on OD pulp, 71C, 165 Min., 10%
consistency
From the results of Example 3 shown in Table III, it m~y be
30 seen that in (H+D) bleaching the chlorine dioxide normally used
after the E/O stage can be replaced to the extent of 50% with
sodium hypochlorite at an equal weight basis. As compared to the
D1 bleaching the (H+D) bleaching even gave slightly higher initial
'
.
- , - ~ ~ .
`~ . '
brightness after the Dl and D2 stage with the oxygen treated pulp
with a 31g6 reduction of the available chlorine usage.
Example 4
A 175 gram sample of unbleached Southern pine kraft pulp
having a Permanganate number of 16 . 3 was treated with 5 . 2%
chlorine in the same manner as in Example 2. The chlorinated pulp
was then neutraii~ed with alkali for an extraction step at 3 . 5%
consistency for 20 minutes at 38C. The Permanganate number of
10 the chlorinated and neutralized pulp (CN pulp) was determined to
be 4 . 3 . The CN pretreated pulp was equally divided into 5 batches
for the first chlorine dioxide bleaching. In order to determine how
much hypochlorite can substitute ClO2 in the first chlorine dioxide
bleaching stage, percent substitution level was increased from 0%
15 (control), 20%, 50%, 70% and 100% (only hypochlorite). After the
Dl or (H+D) bleaching each pulp was further treated with alkali
extraction with addition of caustic and hypochlorite (EH) and
chlorine dioxide bleaching (D2) under the same manner described in
Example 2. The bleaching results of increased percent substitution
20 of hypochlorite in multi-stage bleaching process CND1EHD2 are
shown in Table IV. The changes in Dl brightness and D2 bright-
ness and pulp viscosity by increasing the percent substitution are
illustrated therein. As the table shows, a maximum substitution
level is near 70% of the chlorine dioxide used in the control Dl
25 stage. Up to the 70~6 substitution neither final brightness nor pulp
viscosity was adversely affected.
The Table IV also shows that chemical savings can increase as
percent substitution increases, since at 1983 prices the cost of
sodium hypochlorite is about 5096 less as expensive as the cost of
30 chlorine dioxide at an equal weight basis.
-- 12 --
T AB LE IV
EFFECTS OF SUBSTITUTION OF HYPOCHLORITE FOR CHLORINE DIOXIDE
IN THE FIRST CHLORINE DIOXIDE BLEACHING STAGE IN CNDlEHD2
BLEACHING OF SOUTHERN PINE KRAFT PULP
Dl or (H+D) STAGE
% Sub stitution
of Hypo 0% 30~6 50% 70% 100%
% Hypo 0 0.36 0.6 0.84 1.2
10 % CLO2 1.2 0.84 0.6 0.36 0
Sat. CO2 Water 0.0 40cc 80cc 120cc 160cc
Temp., C 57 57 57 57 57
Time, Min. 90 90 90 60 ' 35
96 Consistency 8 8 8 8 8
EH Stage % NaOH 0.75 0.75 0.75 0.75 0.75
% NaOH 0.50 0.5 0.5 0,5 0.5
D2 Stage, % ClO2 0,4 0.4 0.4 0.4 0,4
20 CN KNO 4,3 4.3 4.3 4.3 4.3
Dl Brightness
(Initial) 66.3 68.9 68.9 66.4 70.8
(% Residual) (0.15) (0.11) (0.06) (0.0) (0.0)
D2 Brightness
Initial 86.9 87.8 87.9 87.2 `86.7
Reverted 83.3 83.6 83.1 82.4 82.0
(% Residual) (0.102) (0.083) ~0.085) (0.072) (0.073)
D2 Viscosity (CPS) 8.0 8.6 8.3 8.3 6.2
Total AvailO CL2
Charged (%) 9.903 9.3 8.93 8.54 7.85
35 Total Avail. CL2
Consumed (%) 9.251 8.81 8.55 8.29 7.76
7~
-- 13 --
EXAMPLE 5
The Northern softwood kraft pulp used in Example 1 was
treated with chlorination and caustic extraction under the following
conditions:
C stage: 5.Z% chlorine based on OD pulp
3.5% consistency, 29C and 45 minutes
E stage: 2 .596 NaOH based on OD pulp
10% consistency, 65 . 5C and 90 minutes
CE-Permanganate number = 4 . 3
The CE pulp was equally divided into three batches. The
first batch was treated with only chlorine dioxide at 1.1% ClO2
charge level based on oven dried pulp as a control first chlorine
dioxide bleaching. The second batch was treated with the (H+D)
bleaching with addition of carbonic acid at chemical charge levels of
0.55% ClO2 and û . 55% hypochlorite (50~6 substitution level) . The
third batch was treated with the (H+D) bleaching without addition
of carbonic acid. All three batch bleachings were carried out in 8%
consistency at 63C. When hypochlorite was added to chlorine
20 dioxide, the retention time had to be shortened from 180 minutes to
f~O minutes because the addition of acid and hypochlorite accelerated
chlorine dioxide bleaching reaction. The three batches were fur-
ther treated with the second caustic extraction and the second
chlorine dioxide bleaching as in Example 1. The results of this
25 experiment are given in Table V.
Comparing the result of batch 1 with that of batch 2, the 50%
substitution of hypochlorite for chlorine dioxide with addition of
carbonic acid gave higher reverted brightness of the resulting
pulp, indicating the (H+D) bleaching improved the brightness
30 stability. As the final brightness and viscosity of the batch 3 are
compared with those of batch 2 j without the acid addition ~ the re-
sulting pulp will have lower reverted brightness, less stable
brightness and lower pulp viscosity indicating some cellulose
degradation during the (H~D) bleaching. From the results, it is
_v~
- 14 -
evident that the acid addition is required to prevent the pulp
viscosity loss and to improve the brightness stability in the (H~ D)
bleaching process.
Table V
EFFECTS OF CARBONIC ACID ADDITION
NORTHEASTERN SOFTWOOD KRAFT PULP
Batch l Batch 2 Batch 3
Dl (H+D) (H+D)
1.1% ClO2 180cc Carbonic 180cc tap
Acid water
0.55% hypo 0 . 55% hypo
0.55~ C1O2 0.55% ClO2
Temp. (C) 63 63 63
Time (Min. ) 180 60 60
Consistency (%) 8 8 8
E2, % NaoH 0.5 0.5 0.5
D2, % ClO2 0.35 0.35 0.35
D2 Bright.
Initial 87.2 87.5 87.6
Reverted 83.0 84.4 83.5
D2 Viscosity, cps 18.7 18.9 17.2
30 EXAMPLE 6
The CE prebleached pulp was prepared from the Northern soft-
wood kraft pulp as discussed in Example l. The CE pulp prepared
was equally divided into three batches for the (H+D) bleaching
followed by E2 and D2 bleaching. The first batch was treated with
35 carbonic acid and 0. 55% hypochlorite followed by 0. 55% ClO2. The
~ ~ :
second batch was treated with the same amount of hypochlorite and
chlorine dioxide but without addition of carbonic acid. The third
batch was first treated with a 0 . 55% ClOz for 120 minutes at
reaction temperature of 63C and, at the end of the C102 reaction,
5 0 . 55% hypochlorite was injected with carbonic acid. After a few
minutes of retention at 63C the chlorine dioxide and hypochlorite
bleached (D+H) pulp was washed. The results of these bleaching
experiments are shown in Table VI.
As may be seen from a comparison of the result of batch 1
lO with that of batch 2, it is again evident that without carbonic acid,
the reverted brightness suffered as shown in Example 5. The
result of batch 3 indicates that in this novel bleaching process the
addition of hypochlorite and aci~ can be reversed; instead of the
addition prior to ClOz addition the hypochlorite and acid can be
15 added at the end of ClO2 bleaching reaction in the same bleaching
stage. The reverse addition may improve further the final bright-
ness of the resulting pulp as shown in the Table VI.
.~2~
- 16 ~
TABLE VI
(H+D) ~D+H)
Batch 1 Batch 2 Batch 3
Mode of
Treatment
Sequence 1.Carbonic Acid 1. No carbonic acid 1. O . 55% and O . 55% hypo O . 55% hypo Cl02
2. 0,55% Cl02 2 . O . 55% C102 2. 2 hours
reaction
at 63 C
3. 0.55%
hypo and
carbonic
acid
E2 (0-5% NaOH)E2 (0~5% NaOH) E2 (0-5% NaOH)
D2 (0~4% C102)D2 (0~4% Cl02) D2 (0~4~ C102)
Brightness
Dl (Initial~ 76.4 7S.6 75.9
D2 (Initial) 89.1 88 . 9 90 . 3
(Reverted~ 86 . 8 82 . 8 ô7.0
EXAMPLE 7
30In this example, different modes of addition of carbonic acid/
hypochlorite/C102 solution in the (H+D) bleaching stage were evalu-
ated. Four different modes were evaluated:
Mode 1. Simultaneous addition of carbonic acid and
hypochlorite solution followed by 30 minutes retention
35at room temperature and then additiorl of C102 solu-
-- 17 --
tion without washing step after the hypochlorite
addition and retention.
Mode 2. The same mode of addition as Mode 1 but with a
washing step prior to the ClO2 addition.
Mode 3. Simultaneous addition of carbonic acid and hypo-
chlorite solution and ClO2 solution while agitating.
Mode 4. Sequential addition of carbonic acid first and hypo-
chlorite solution and ClO2 solution last without
retention time and washing between each addition.
The CE pulp sample was prepared by the same manner as in
Example 1.
All four (H+D) bleaching experiments were carried out at 63C
in 10% consistency but with varying retention times. The effects of
different addition modes were monitored by reading initial ancl
15 reverted brightness of the resulting pulp after the (H+D) stage.
The results are shown in Table VII. From the result, it is obvious
that this novel bleaching with a combination of hypochlorite and
chlorine dioxide does not require x etention in time or heating and
washing step between the hypochlorite addition and chlorine dioxide
20 addition, and that the addition of acid, hypochlorite and chlorine
dioxide solutions can be added simultaneously or sequentially.
However, the sequential addition may be more effective than the
simultaneous addition.
-- 18 --
Mode of Addition
1 2 3 4
- Carbonic Acid + - Carbonic Acid ~ Simultaneous Sequential
hypo (simult. ) hypo addition of addition
carbonic acid/
- 30 minutes - 30 minutes hypo/ C1O2 ( Carbonic acid)
retention at retention
room temp. - Hypochlorite-
- Washing ClO2)
- No washing
- ClO2 addition
- ClO2 addition
- no retention
and washing
a.fter hypo
,
Final pH
2.0 2.3 2.5 2.3
ClO2 Residual
(titre) 0 . 3cc 0.2cc 0.3cc 0.3cc
Ret . time in (H~D )
stage 120 min. 120 min. 80 min. 90 min.
25 (H+D) Brightness
Initial
76.6 75.1 74.1 75.2
Reverted
73.2 72.6 71.5 72.6
EXAMPLE 8
A CE prebleached pulp was prepared in the same manner
described in Example 5 for Northern softwood kraft pulp. The CE
pulp was equally divided into three batches for the first chlorine
7~
-- 19 --
dioxide bleaching stage in multistage bleaching CED1ED2. The first
batch was bleached with chlorine dioxide only at a charge level of
1.1% ClO2 based on OD pulp at 63C for 180 minutes. The second
batch was bleached with a combination of carbonic acid and hypo-
5 chlorite and chlorine dioxide at a 5096 substitution of hypochloritefor chlorine dioxide (0.55% hypochlorite plus 0.55% ClO2) at the
same temperature of the first batch but with reduced retention time
due to its faster bleaching reaction. The third batch ~as treated
with the same chemical level of the second batch but at lowered
10 temperatures. The dioxide bleaching temperature was lowered from
63C to 54C. After the D1 or (H+D) bleaching those batches of
pulp were further treated in the E2 and D2 stage by the same man-
ner in Example 1. The result of lowered temperature in the (H+D)
stage is shown in Table VIII. From the result it is obvious that in
15 this novel (El+D) bleaching the bleaching temperature can be
lowered by 9C from 63C to 54C without adversely affecting the
final brightness of the resulting pulp of multistage bleaching
CE(H+D)E2D2 ~
~ ` .
- 20 -
TABLE VIII
Northern Softwood Kraft Pulp
CE pulp ( CEKnO 4.2)
Dl or (H+D) Batch 1 (Control) Batch 2 (H~D) Batch 3 (H+D)
% ClO2 1.1 0.55 0.55
~ Hypo 0.0 0.55 0.55
Temp. C 63 63 54
Retention
time (Min)180 120 150
Residual,
titre (ml) 1. 0 none none
pH 1.7 2.4 2.6
E2, ~ NaoH0.5 0.5 0.5
D2, % ClO20.4 0.4 0.4
Brightness
D1 (Initial) 77 . 6 72 .1 73 .1
D2 initial90.1 90.1 90.1
D2 reverted 85. 6 85.5 85.4
EXAMPLE 9
Effects of different temperature of the (H+D) stage were again
investigated in the multistage bleaching process CN(H~D)EHD2 using
Southern pine kraft pulp. A chlorinated and alkali neutralized pulp
30 was prepared by the same manner in Example 4. The K number of
the CN pulp was 5 . 2 . The CN pulp was equally divided into four
batches. Each batch was bleached with the (H~D) bleaching at
different temperatures, 48C, 54C, 60C and 71C. After the
(H+D) bleaching, all three pulps were further treated with second
35 caustic extraction with hypochls:)rite (EH~ and second chlorine
7~
-- 21 --
dioxide bleaching under the same conditions. The results are
shown in Table IX. From the result, it is again evident that in the
novel (H+D) bleaching, its bleaching termperature can be lowered to
48C without any adverse effect on the final brightness and pulp
5 viscosity from the normal temperature range of first chlorine dioxide
bleaching, 60C - 71C. The lowering of bleaching temperature
means a substantial energy savings at the bleach plant.
TABLE IX
Batch 1 Batch 2 Batch 3 Batch 4
Temperature in
(H+D) Stage, C 48 54 60 71
Retention time, min. 180 120 100 80
Consistency, % 8 8 8 8
End pH in (H+D) 2.25 2.25 2.2 2.1
20 % Residual in tH+D) 0.0 0.0 Q.0 0.0
(H+D) Brightness 73.2 70.1 71.8 73.2
( Initial )
25 D2 Brightness,
Initial 87.9 87.4 87.9 88.3
Re~erted 84.3 83.9 84.0 84.9
D2 Viscosity, cps 13.3 14.0 13.5 13.7
EXAMPLE 10
Using a Northern softwood kraft pulp the CE prebleached pulp
sample was prepared under the following conditions:
C stage: 5.2% chlorine based on OD pulp,
,
-- 22 --
3 . 5% consistency, 29C and 45 minutes
E stage: 2.5~6 NaOH based on OD pulp,
11% consistency, 71C and 90 minutes
The prepared CE: pulp was e~ually divided into six batches for
the (H+D)E2D2 bleaching using six different acids in the (H~D)
bleaching stage. The six different acids evaluated are carbonic
acid (saturated CO2 water), acetic acid (10% concentration), nitric
acid (10%); hydrochloric acid (lN solution), sulfuric acid (4N
solution) and phosphoric acid (10g6 solution). Each acid was added
in an amount which pH of the (H+D) bleaching reached in the range
of 2 . 0-2 . 5 . The temperature and retention time were held constant
at 57C and 60 minutes respectively. After the (H+D) bleaching
each pulp was further treated with 0 . 5% NaOH in E2 stage and
0 . 35% ClO2 in D2 stage under the same conditions described in
Example 1. The results of dif~erent acids are given in Table X.
From the results shown it is obvious that in the noble (H+D)
bleaching process besides carbonic acid other organic and inorganic
acid can be used as long as the end pH reaches in ihe range of
2 . 0-2 . 5 . Regardless of the acid used, the same final brightness
can be achieved without affecting pulp viscosity.
However, considering practical application of this novel (H+D)
bleaching process under mill environments, weak acids having ion-
ization constant of 4-7 are preferred such as acetic acid,
phosphoric acid and carbonic acid in order to avoid any danger of
acid cellulose degradation in case of overfeeding strong acids such
as sulfuric, nitric and hydrochloric acids. Among the weak acids
evaluated, carbonic acid i5 preferred since it has the following
advantages over other acids:
1. Carbonic acid can be readily prepared at the mill by
injecting CO2 gas into mill water.
2. It does not impose corrosion problems related to chloride
and sulfate ions.
3. It does not impose any hazard in working environments
because CO2 gas is not toxic.
.
- 23 --
- 4. Any overfeedin~s will not cause an undue cellulose
degradation .
5. It may be less expensive than other acids in neutrali~ing
or acidifying alkaline pulp slurry.
TABLE X
Bleaching Sequence: CE1(H+D)E2D2
End pH of (H+D) D2 D2
Acid (H+D) Stage Brightness Brightness Viscosity(cps)
Carbonic 2.45 69.1 88.3/84.7 20.4
Acetic 2.50 68.8 88.1 / 84.3 20.5
15 Nitric 2,30 69.3 88.5/84.5 21.1
Hydrochloric 2.30 68.5 87.9/84.0 21.2
Sulfuric 2.25 68.7 88.7 / 84.8 20.4
Phosphoric 2.30 68.5 88.7 / 84.9 20.7
It is apparent that other variations and modifications may be
made without departing from the present invention. Accordingly, it
should be understood that the forms of the present invention des-
cribed above are illustrative only and not intended to limit the
scope of the invention as defined by the appended claims.
.
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