Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1. BACKGROUND OF THE INVENTION
(i) Field of the Invention
The present invention relates to an improvement in
a bleaching process and apparatus and in particular,
relates to an improved method and appartus for decreas-
ing the charge of chemical required in a bleaching
extraction stage.
(ii) Description of the Prior Art
The recycling of some of the extraction effluent
during chemical pulp bleaching is a common practice to
conserve heat and to displace the acidic chlorination
liquor. The recycled extraction stage liquor is added
to the chlorinated pulp as it leaves the chlorination
stag~ washer via the washer showers and this recycled
liquor ls carried with the pulp back into the extrac-
tion stage (~-listed, J.~. and Nelson, Jr., G.G. in "The
sleaching of Pulp", R.P. Singh, Ed., 3rd Edition, TAPPI
Press, Atlanta, 1979, p. 393; Histed, J.A. and Nicolle,
F.M.A., PUlp Paper Mag. Can. 7~ t5) 'rl71 (1974);
Wartiovaara, I., Paperi ja Puu 62 ( 5) 319 ( 1980); and
wartiovaara, I., Pulp Paper Can. 81 (7) T167 (1980)).
It is also known in the prior art (Burkart, L.~.,
Paper Trade J. 156 (2) 33 ~1972) and Azad~ A.M. and
Burkart, L.F., Tappi 59 (4) 140 (1976)) to use recycled
effluent fortified with sodium hydroxide as the extrac-
ting liquor for the first eXtraCtion stage. This is
found to decrease the amourlt of sodium hydroxide
required in the extraction stage by about 16~.
ti
2. S~MMARY OF THE INVENTI~N
(i~ Aims of the Invention
.
It is an object ot the present invention to provide
an improved process and apparatus for decreasin~ the
amount of chemical required in an extraction stage of a
chemical pulp bleaching sequence.
(ii) Statement of the Invention
.
According to the present invention, there is pro-
vided an additional washing step before the extraction
stage begins.
In yreater detail~ in a bleachin~ process wherein
a pulp is sequentially subjected to a chlorination, a
chlorination washing step, a mixing step with an alka-
line solution (usually sodium hydroxide and steam), an
extraction step, followed by an extraction stage wash-
ing step, there is provided ttle improvement which
includes the steps of subjecting the pulp to an addi-
tional washing step between the chlorination washing
step and the mixiny step. Effluent frorn the extraction
stage washing step is recycled to the pulp i~mediately
after the chlorination washing step. E~fluerlt from the
additional washing step is recycled for use in the
chlorination washing step.
There is also provided an improved bleaching appa-
ratus which includes an additional washer between a
chlorination washer and a mixer for mixing steam and
the al~aline solution with the pulp before it enters an
extraction tower. There are provided means for direct-
ing effluent from the additional washer to the chlorin-
ation washer and means for directing the effluent from
the extraction washer to the pulp at a point between
the chlorination washer and the additional washer.
3. BRIEF DESCRIPTION OF THE DRAWINGS
Having thus generally described the invention,
reference will be made to the accompanying drawings
illustrating an embodiment thereof, in Which:
FIGURE 1 is a simplified flow diagram for a recy-
cling system in a conventional bleach plant having a
chlorination stage followed by an extraction stage.
E'IC~URE 2 is a flow diagram similar to Figure 1
illustrating a verSion of the improved system of the
present invention.
4. DESCRIPTION OF PREF~RRED EM~ODIM~N1~
(i) Detailed ~escription of F`i~. 1 and 2
Chlorination dcscribed hereirl includes the various
modifications of the chlorination stage where chlorine
dioxide replaces some or all o~ the chlorine. Chlorine
and chlorine dioxide are examples of oxidising cherni~
cals that can be used before an extraction stage. Any
extraction stage preceded by an oxidising Step can be
improved ~y the application of this invention.
In greater detail, as shown in Figure 1, a conven-
tional chlorination bleaching system having a recycling
step includes a chlorination tower 10, piping 12 lead-
ing to a chlorination washer 14~ piping 16 leading to a
mixer 18 wherein the alkaline solution and steam are
mixed with the pulp, a further transport pipe 20 to
extraction tower 22 and a transport pipe 24 which leads
3'~3~
to the extraction stage washer 26. Means, 28, are pro-
vided for returning effluent from the extraction stage
washer 26 to the chlorination washer 14.
In the present invention lll~strated in Figure 2,
similar reference numerals are employed for similar
components. However, it will be noted that there is
provided an additional washer ~o, between chlorination
washer 14 and mixer 18. In this instance, the effluent
from extraction washer 26 i~ recycled via line 28 to
the pulp at a point between washers 14 and 30. A
source of turbulance at this point is beneficial to
ensure proper mixing. Effluent from additional washer
30 is recycled to the chlorination washer 14 via line
32.
The liquid used for the showers in the additional
washer 30 is introduced via line 34 and may be provided
from an extraction effluent or water source (not
shown). The washing step is important because it
removes Na~C03 an~ C02 formed ~alony Wittl NaCl) in
the reaction of chlorinated pulp with extraction efflu-
ent. If NaHC03 and C02 remain with the pulp they
useLessly consume a large portion of the chemical
charge in the extraction stage.
(ii) Examples Illustrating the Practice of the
Preferred Embodiments
EXAMPLE 1
. _
Example 1 illustrates the sodium hydroxide savings
that are possible by washing pulp treated with extrac-
tion effluent.
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1;2~4396
A spruce kraft pulp (Kappa No. 27.5) was chlorina-
ted, well-washed and pressed to 20% consistency. It
was then diluted to l0~ consistency with effluent from
the first extraction stage. This provided 5 mL of
recycled effluent for every l g o.d. of pulp. This 5
mL 9-l, however, could have been added by displacing
the liquor associated with the pulp or by just simply
diluting to lower consistency. The extraction efflu-
ent, at 60C, remained in contact with the pulp for 5
minutes. The pulp washed with water and then extracted
with aqueous alkali. A second sample was chlorinated,
washed well, and pressed to 20% consistency. It was
then diluted to 10% consistency with extraction efflu-
ent from the first extraction stage. The extraction
effluent, at 60c, remained in contact with the pulp
for 5 minutes and then alkali was added to it and the
extraction completed. A control sample of washed,
chlorinated pulp was extracted without any pretreat-
ment. All 3 pulps were then further bleached using a
DED partial sequence.
As Table I shows, when effluent recycle and wash-
ing were used according to the invention, final bright-
ness of 89~ was easily obtained even though the chemi-
cal consumption in the El stage was decreased by one-
third. The conventional type of recycle wherein the
recycled effluent is not washed out of the pulp provid-
ed no advantages in terms of CE Kappa number or NaOH
usage.
3~
TABLE I
Type of NaOH Charge
El in the
Effluent Extraction CE ISO Brightness,
Recycle Stage, % on Kappa %
o.d. Pulp No.Dl D2
.
No recycle 3.0 5.481.0 89.5
2.0 6.176.7
1.0 9.056.5 8g.6
Conventional 3.0 5.5 81.4 89.6
Recyle 2.0 5.876.6 87.9
(with no wash) 1.0 8.4 60.7 ~5.5
Recycle 3.0 5.182.2 89.9
Followed by 2.0 5.3 81.7 89.6
washing 1.0 5.879.6 88.7
,. . _.,
Unbleached pulp: ~lack spruce kraft pulp; Kappa
No., 27.5
C-stage : 5.5% C12 on o.d. pulp, 45
min at 25C 3.5~ consistency
E-stage : 90 min at 70 C; 10
consistency
Dl-stage : 1~ ClO2 and 0.55% NaOH on
o.d. pulp; 3 h at 6~C, 10%
consistency
E2-stage : 1~ NaOH on o.d. pulp; 1 h at
60C 10~ consistency
D2-stage : 0.5% NaOH on o.d. pulp; 3 h
at 60'C; 10~ consistency
The extraction effluent used foe recyclin9 was produced
by extracting the same chlorinated pulp with 3% NaOH on
o.d. pulp.
EXAMPLE 2
. . .
Table II shows that the alkali savings whiCh were
shown to be possible in Example 1 are maintained when
the recycling system approaches steady state.
In Run 1, a spruce kraft pulp (Kappa No. 27.5) was
chlorinated, well washed and extracted (3.0% NaO~l on
o.d. pulp) without any pretreatment. In Run 2, a
second sample of the same pulp was chlorinated, washed
well and pressed to 20% consistency. It was then
diluted with extraction effluent from the extraction
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lZ9~3~6
stage of Run 1. The extraction effluent, at 60C,
remained in contact with the pulp for 5 minutes.
The pulp was washed with water and extracted with only
2% of NaOH on o.d. pulp. In Run 3, a third sample of
the pulp was chlorinated, washed well and pressed to
20~ consistency, It was then diluted with extraction
effluent from Run 2. Again the pulp was washed with
water and extracted with only 2% NaOH on o.d, pulp.
This procedure of using the effluent from the preceding
run was repeated. Table II shows that the CE Kappa
number is maintained even though the NaOH charge was
decreased from 3.0 to 2.0~ on o.d. pulp in Runs 2, 3
and 4.
In Run 5, a sample of the pulp was chlorinated,
well wa~hed and extracted with only 2% NaOH on o.d.
pulp. The CE Kappa number was higher than the value
value obtained when using effluent recycle and wash
according to the invention.
Clearly a charge of 2~ NaOH on o.d. pulp used
according to the invention is equal to ~ NaOH used in
the conventional manner.
TABLE II
Na~ll Charge
Source of in the
Type Run Effluen t Ex t rac t ionCE
o~ No. for staye, ~ on Kappa
Process ~ecycleo.d. Pulp No.
Conven-
tional 1 No recy. ~.0 4.3
Recycle/
Washing 2 Run 1 2.0 4.3
Recycle/
Washing 3 ~un 2 2.0 4.1
Recycle/
Washing 4 ~un 3 2.0 4.0
Conven-
tional 5 No Recy. 2.0 5.2
. .
Conditions: As listed in footnote to Table I.
EXAMPLE 3
This example illustrates that changes in the time
and temperature of treatment of chlorinated pulp with
recycled extraction effluent do not effect the outcome
of the procedure.
A spruce kraft pulp (Kappa No. 27.5) was chlorina-
ted, well washed and presssed to 20% consistency. It
was then diluted to 10~ consistency with effluent from
the first extraction stage. A time and temperature
combination for this reaction between the chlorinated
pulp and the recycled extraction effluent was chosen.
The pulp was washed with water and then extracted with
2.0% NaOH on o.d. pulp. The experiment was repeated
with different time and temperature combinations for
the reaction between the chlorinated pulp and the
recycled extraction effluent.
1~¢~ 6
Table III shows that the useful reaction between
the chlorinated pulp and the recycled extraction efflu-
ent takes place within 30 seconds at temperatures of
25C and higher. The significance of this is that the
reaction can be done in a pipeline, and no special
reaction vessel would be necessary.
TABLE III
Recycled Extraction Effluent CE
Temperature,Time of Contact, Kappa
C min.__ No.
0.5 4.0
1.0 4.0
5.0 4.1
10.0 3.9
c- and E=staye conditions: As in footnote to Table I.
EXAMPLE 4
The addition of a small amount o~ chlorine dioxide
to the pulp in the chlorination stage is a common prac-
tice for minimizing the damage done to cellulose during
chlori na t ion .
Table IV shows that the CE Kappa number oE conven-
tionally treated pulp is not much affected if 10~
C102 substitution is used. Moreover, by treating the
chlorinated pulp according to the invention a reduction
in E-stage NaOH to 2.0% or less can be achieved regard-
less of whether C102 is used or not.
g
3~6
TABLE~:_ IV
NaOH Charge
Cl 2 i n the
Type of Substitution Extraction CE
Process in C-Stage Stage, ~ on Kappa
% o.d. pulp No.
Conventional o 3.~ 4.3
Recycle/Washing o 2.0 4.2
Recycle/Washing 0 1.5 4.2
Recycle/Washing 0 1.0 4.1
Conventional 10 3.0 4.1
Recycle/Washing 10 2.0 4.0
Recycle/Washing 10 1.5 4.3
Recycle/Washing l0 I.0 4.4
Unbiëached pulp: Black spruce kraft pulp; Kappa No.
27.5
C-stage ~ 5~5~ ~12 on o.d. pulp: 4S min at
25UC; 3.5% consistency
CD - ~tage 5.0e C12 and 0.2% C102 on
o.d. pulp: 45 min at 25 ~ ~.5
con~istency.
E-Stage : so min at 70~C; 10~ consistency.
The extraction effluent used for recycl.in~ was produced
by extracting the same chlorinated pulp with 3~ NaOH on
o.d. pulp.
5. SUMMARY
It will be understood that the above-described
embodiments are for purposes of illustration only and
that changes and modifications apparent to those
skilled in the art may be made.
It will also be apparent that a displacement
bleaching system could be used to perform the steps of
the invention.
The concept of using the extraction ef1uent as
described will not be affected by chemical addition to
the extraction stage. Oxygen, peroxide or hypochlorite
addition to the extraction stage ~,ill not alter the
principle of decreasing the sodium hydroxide charye by
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` ~I,'~g4~396
washing after recycling and before the extraction
s tage .
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