~03~
FIELD OF THE INVENTION:
This invention relates to an improvement in a
method for bleaching chemi.cal pulp by using an ozonous gas
according to a multi-stage system. -
`.
-
BACXGROUND OF THE INVENTION:
As is well known, a bleaching step is essential
in the pulp manufacturing process for obtaining a desired
brightnessof the produced pulpo ~ For such bleaching step,
there is generally em*loyed a mult~i-stage bleaching method
in which the pulp is treated ln multiple stages by using
suitable chemicals such as chlorine, chlorine dioxide,
hypochlorite, hydrogen peroxide,~etc.~ Such method, however,
has the possibility of inducing the so-called water pollution
problem in disposal of waste liquor produced from such t.reat-
ment and the possibility that organic chlorine compounds
as the reaction products are poisonous to fishes living
.
.in the water to which the waste liquor is released, so that
a vast expenditure is necessitated for the instaIlation of
the waste liquior treating facilities and operation thereof.
Thereforej studies have been made in the industry
in search of a bleaching method which has little likelihood
of bringing about the pollution problem by waste liquor.
Particular attention is riveted these days to the pulp
bleaching method using an ozonous gas, and efforts are
~3~
being exercised for industrialization of this method~ It
has been known since long that pulp can be bleached by
ac-ting an ozonous gas to pulp, but it has been considered
hardly possible to put this method to practical use on an
industrial scale for the reasons that when pulp is bleached
by this method to such a degree ofbrightneSSaS acceptable
for commercial use, (1~ an enormous amount of ozone is
consumed, and (2) the physical and chemical properties of
the pulp are intolerably deteriorated.
Various efforts have been made in the industry
for obtaining the techniques which can overFome the above-
said two seriou~i defects. In 1974, Soteland reported the
fact that when an ozonous gas is acted to pulp, there is
produced a muconic acid derlvative as a result of ozone
oxidization of lignln, and this is responsible for the
consumption of ozone ~N. Soteland: Pulp Paper Mag. Can~,
T 153 (1974)]. In recent years, there is suggested a
method o bleaching a chemical pulp of a wet state with a
pH of 2.0 6.0 by using an ozonous gas containing ozone in
a concentration within the range of l to 15 mg/Q, for example,
United States Patent No. 2,466~633 discloses a method of
bleaching a cellulosic pulp having a pulp concentration of
25 - 55% and a pH of 4 - 7 by using an air containing about
l/2 % by weight of ozone and United States Patent No.3,451,888
a method of bleaching a paper-making pulp which is in the
fcirm of an aqueous slurry having a dry consistency of 30 to
-- 2 --
~3~
65% by using a gaseous mixture of ozone containing 2.5 - 18
mg O3/Q and having a moisture content near 100%. Canadian
Patent No. 966,604 discloses a method of bleaching Kraft
wood pulp by a sequence with ozone and an alkaline peroxide
solution. Furhter in 1975, Rothenberg et al reported a
multi-stage pulp bleaching process in which an ozonous gas
is acted to pulp and then the thus treated pulp.is washed with an
aqueous sodium hydroxide solution or hot water [S. Rothenberg
et al. Tappi~vol~ 58, No.8~la2(l975)]land in 1976, Kobayashi
et al.reported that by employing a multi-stage bleaching
process (acting an ozonous gas to pulp and then washing
pulp with water)l it is possible to lessen the ozone con-
sumption as c~,mpared with the conventional "single~stage"
bleaching method where an o:zone gas ~s acted to pulp
contlnuously until a~desired brightnessis obtained, and it
is also possible with such multi-stage bleaching process to
control, to some extent, the reduction of relative .viscosity
of the bleached pulp [T. Kobayashi et al.: Japan Tappi,
Vol. 30, No, 6 (1976)].
Notwithstanding the fact that such multi-stage
ozone bleaching method, as compared with the previous single-
stage bleaching method, is evidently lessened in ozone
consumption and also capable to arrest to some extent the
deterioration of the physical and chemical properties of
the obtained bleached pulp, such multi-stage bleaching
method is not yet available for practical use on an
-- 3 --
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industrial scale~ This is supposed due tci the facts that in
practicing the multi-stage ozone bleaching process bn an
industrial scale, a huge cost is necessitated for producing
ozone required in the process, and it is extremely difficult
to fully utilize the costly ozone by perfectly reacting such
ozone with the puIp, resultIng in a very poor economy.
SUMMARY OF THE INVENTIONo
The object of this Inventlon, therefore, is to
provide an improved puIp bleaching method by use of ozone~
which method is capab~le of bleaching a chemical puIp with
increased bleaching efficiency by ozone without deteriorating
the physical propertIes of puIp, particularly relative
viscosity thereof, and which can also lend itself to
industrial practice with high economical effect.
This and other objects of this invention will
become more apparent from the following detailed description
of the invention.
As a result of extensive studies for realizing
industrialization of the multi-stage pulp bleaching process~
with ozone that has been strongly desired in the industry,
we have found that, in bleaching the pulp of a wet state
with a pH of 2.0 - 6~0 according to the multi stage system
by using an ozonous gas containing ozone in a concentration
within the range of 1 to 15 mg/Q, oæone is most effectively
consumed when the exhaust gas released in the bleaching
process from the second stage of bleaching onward is reused
in the first stage bleaching step. This invent;on was
attained on the basis of this finding. ,-
Thus, according to this invention, there is
provided a method of bleaching a chemical pulp of a wet
state with a pH of 2.0 - 6.0 in multiple stages wherein
the steps of acting a gas containing oz0ne in a con-
centraticn within the range of 1 to 15 mg/Q to the pulp and
then washing the thus treated pulp are repeated successlvely,
which method including using an exhaust gas released in the
bleaching process from the second stage onward in the first
bleaching stage.
BRIEF DESCRIPTION OF T~E DRAWINGS:
In the accompanying drawings: ~
Fig. 1 is a flow sheet illustrating an embodiment
of this invention where the pulp is bleached by introducing
an ozonous gas in a series relation into the respective
bleaching steps exclusive of the first stage bleaching
step; and
Fig. 2 is a flow sheet illustrating another
embodiment of this invention where the pulp is bleached by
introducing an ozonous gas in a parallel relation into the
respective bleaching steps exclusive of the first stage
bleaching step.
3~
DETAILED DESCRIPTION OF THE INVENTION:
As is well known, ozone used for bleaching of the
pulp can be produced from oxygen or air in various ways, for
example, according to a silent dischaxge system, photo-
chemical reaction system or plasma discharge system. It is
possible in this inveniton to use ozone produced according
to any o~ such systems, but~for use in the process of this
invention, such ozon~needs to be contained in a gas such
that the ozone concentration will be within the;range of 1
: : :
to 15 mg/Q.
According to the method of this invention,~ the
steps of actin~ an ozonous gas~with the above-said range of~
ozone concentration to the pulp of a~wet state with pH 2.0
to 6.0, and then washlng;the~ thus~acted pulp a~re~ repeated
successively. Use~of the "pu1p of~a wet state with pH 2.0
to 6.0" is essential for making the ozone action to the pulp
most effective. Such pulp of this state can be obtained by
impregnating the stock pulp with an acidic aqueous solution
adjustea to pH 2.0 to 6.0, preferably an aqueous solution
:: :
of nitric acid or nitrate, and then adjusting the moisture
content to the range of about 50 to 300~.
It should be noted that if the pH value of the
solution is less than 2.0, the physical and chemical proper-
ties of the obtained bleached pulp prove intolerably poor,
~ J~
while if the pH vanue is greater than 6.0, it becomes
impossible to attain any significant decrease of ozone
consumption.
Any suitable known method may be employed for
acting an ozonous gas to the pulp. For instance, it may be
accomplished by putting the pulp impregnated with an acidic
aqueous solution into a closed chamber and then introducing
an ozonous gas thereinto, or by introducing the pulp
impregnated with an acidic aqueous solution into a closed
chamber already CGntalning an ozonous gas~ It is~also
possible to first put the pulp into a closed chamber, then
spray a predetermined amount of an acidic solution thereinto
to have the~pulp impregnated with the solution and then
introduce an ozonous gas~
This bleaching process is usually carried out at
room temperature and under atmospheric pressure~
Washing of the thus treated pulp (the step of
washing-extraction present between one stage of ozone-
treatment and another stage of ozone-treatment in the
mùltiple stages) may be also accomplished in a usual way
by using an alkaline aqueous solution such as a dilute
sodium hydroxide solution or water. In case of using the
former, it is preferably that sodium hydroxide is present
in an amount of approximately 1 wt~ based on the weight
of pulp~ The purpose of such washing is to remove out of
the system the unstable, easily oxidizagle intermediates
~3~
produced from ozone, so that the purpose can be accomplished
by making sufficient washing with running water or such.
The above-said bleaching method of this invention
is repeated at least more than twice, preferably more than
thrice, until the pulp is bleached to the desired degree
of brightness. The reason why more than three times of
repetitlon is preferable than two times of repetition is
that the more the number of repetition, the more amount of
easily oxidizable intermedlates produced after reaction
with ozone is removed out of the reaction system and hence
the ozone consumption is accordingly lessened, resulting in
an increased ozone effect. The number of such repetition
may be suitably decided by taking into account the proper-
ties of the pulp used, plant installe~tion conditions and
other factors.
In the present invention, in bleachiny the chemical
pulp of a wet state with a pH of 2.0 - 6. n in the multiple
stages as stated above by using an ozonous gas containing
ozone in a concentration within the range of 1 to 15 mg/Q,
an exhaust gas released in the bleaching process from the
second stage onward is used in the first bleaching stage.
In industrial bleaching of pulp, it is imperative to use
expensive ozone with the lowest possible loss. In this
consideration, we minutely measured and examined the bright-
ness increase rate obtained per unit amount of ozone and
ozone concentration in the gas released from each bleaching
~3~
step after a predetermined period of time in case of using
ozonous gases with various ozone concentrations in the
respective stages of the bleaching process, and as a result,
we could obtained the interesting new findings such as ,
shown in Table 1 below.
_ g _
<IMG>
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As apparent from the data given in the above Table
l, the unbleached pulp withbrightnessof 30 points ls
increased inbrightnessby 1.53 to 1.58 points in the first
stage of bleaching treatment with ozone of 0.1 wt% based
on the pulp weight, by using an ozonous gas with ozone
concentration of 10 to l5 mg/~, and abrightnessincrement
of 6.69 to 4.89 points is obtalned in the second stage of
bleaching. However, in the third stage bleaching treatment,
the brightnessincrement wlth ozone of 0.l wt% based on the
pulp weight varies greatly according to the ozone concentra-
tion in thé ozonous gas used: 3.5 points wlth ozone ~
concentration of 15 mg/~, 7.2 points with ozone concentxa-
tion of 10 mg/~ and 10.9 polnts with ozone~concentration
of 5 mg/Q. These experimental results testify that a ~.
:
higher bleaching effect is provided by using an ozonous
gas with a lower ozone concentration.
On the other hand, the ozone consumption calculated
on the pulp base; is higher than 97.8% on the average with
the 20-minute treatment in the f.irst stage bleaching, and
in the case of S mg/Q~ozone concentration, ozone is perfectly
consumed (100% consumption). The ozone consumption was 46%
in the second stage bleaching and 25.4% in the third~stage.
Thus, an extremely high ozone consumption in the bleaching
treatment of unbleached pulp was confirmed. This is
attributable to the reason that ozone is quickly reacted
with a small quantity of impurities such as lignin contained
~3~ 9
in the unbleached pulp. These experimental results also
point to the tendency that a hiyher bleaching effect is
provided from a lower concentration of ozone after lignin
has been eliminated to some extent.
Generally, in pu1p bleaching with ozone, it is
considered that relative viscosity of pulp decreases
extremely with increase of brightness under the action
of ozone. It has been reported in literatures, e g.
C. 5chuerch,: Japan Tappi, Vol. 21, No.l, 13 tl967),
that decomposit1on of lignin and degradation of cellulose
in pulp take place simultaneously in ozone bleaching.
We have found, however, from our close experimental
examinations that lignin and other impurities contained
in unbleached Kraft pulp are most reaclily reacted with
ozone, so that as far as these impurities exist, ozone
is substant~ally consumed by them and there occurs l1ttle
degradation of cellulosic material when a gas containing
ozone in a low-concentration~is used.
However, if consideration 1S made from the view-
point of consumption of introduced ozone in a low concen-
tration, the following facts are noted. That is, although
a high ozone consumption (98%) is seen in unbleached pulp
introduced into the first stage of bleaching process, the
consumption drops excessively (46%) in the second and
ensuing stages, and in the final stage of bleaching where
pulp reaches to the brightness of 70 - 80 point, the
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~3~9
reaction rate of ozone is extremely low (25.4%) even if an
ozonous gas with a low ozone concentration is used, and a
lot of unreacted ozone remains even after the colored
substances have been decomposed and pulp brightness has
been increased.
Thus, if the ozone concentration in the introduced
gas is 10 mg/Q in the first stage of bleaching, 15 mg/Q in
the second staye and 5 mg/Q in the third stage and the
pulp is respectively treated by these gases for 10 minutes
at a rate of 1 Q/min., ozone is consumed at the rate of 98%
in the first stage, 54% in the second stage and 25.4% in
the third stage, so that, as obvious from the following
formula, the total ozone consumption is only about 50%,
and as much as about 40% of unreacted ozone is left in the
discharged gas.
~100 mgx 0.98+ 150mg x0.54+ 50mgx 0.25)X1oo ~ 63 8%
100 mg+ 150mg+ 50mg
.
Release of such rich discharge gas into the
atmosphere leads not only to air pollution but also to
formidable loss of expensive ozone. Even if such discharge
gas is reduced into an ozone generator to increase concen-
tration, it needs to perfectly remove moisture, volatile
organic compounds and so on in discharge gas for reutili-
zation thereof, and this might result in a reduced ozone
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34~
generation efficiency.
According to the present invention, the ozone-
loaded gas discharged from the second and ensuing at;least
one stage of bleaching process is guided back into the first
stage bleaching step where the reaction rate is extremely
high, so as to reduce the ozone conce~tration in discharge
gas to substantially zero thereby to realize almost perfect
utilization of ozone while preventing atmospheric pollution
from such gas.
Thus, the~present~invention has accomplished a
non-pollution system;for the multl-stage ozone pulp bleaching
process in which discharges gas from the second and ensuing
at least one step is guided back into;the first stage bleaah-
ing step and reacted with unbleached pulp, so as to release
the gas whlch is substantially free of ozone. In the steps
of the second and ensulng at least one stage, it is possible
to gradually decrease the ozone concentration in the ozor.ous
gas.
In this invention, the ozonous gas from the ozone
generator may be supplied in a series relation to the
respective steps after the second stage bleaching, or
alternatively such ozonous gas may be distributed in a
parallel relation to the respective steps so that the
gas discharged from the respective steps will be led back
into the first stage bleaching step, therehy to have
ozone in the gas substantially perfectly consumed by the
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Lq334~
pulp~ Alternatively, the oæonous gas from the ozone
genarator may be countercurrently introduced to the
running direction of pulp in such a manner as that the
ozonous gas is firstly fed into the last bleaching step,
the exhaust gas released from the last bleaching step is "
fed into the bleaching step preceding thereto, and so the
exhaust gas released from all the bleaching steps excepting
for the first bleaching step is fed into the first
bleaching step, thereby to have ozone in the gas substantially
perfectl~ consumed by the pulp.
The process of this invention LS now described
in ~urther detail by having reference to the flow sheets
in the accompanying drawings.
Referring first to~Fig. l, there lS shown a
preferred form of the pulp bleaching system according to ~
this invention including three bleachers l, 2 and 3 and
four pulp washers A, B, C and D which are arranged alternately
in a series relation as shown. Ozone from an ozone generator
5, to which air ~ is introduced, is supplied into the first
stage bleacher l alone through an ozone concentration
adjuster 6. It is also supplied into the second stage
bleacher 2 through a different ozone concentration adjuster
7. The discharge gas from the second stage bleacher is
led into the third stage bleacher 3 and discharge gas from
the third stage bleacher 3 is guided into the ozone concentra-
ion adjuster 6 for the first stage bleacher l.
- 15 -
By using this system, pulp 8 is first fed into
the pulp washer A and thereby washed and prepared into
unbleached pulp withbrightnessdegree of 30 points,
moisture content of 300% and pH of 4.0, and this un-
bleached pulp is led into the~first stage bleacher i and
then successively passed through the ensuing washers and
bleachers to repeat the washing and bleaching operations.
Air 4 supplied into the ozone generator 5 is made into an
.
ozonous gas with ozone concentration of approximately
30 mg/Q, so this gas,~after diluted to lO to~l5 mg/Q
ozone concentration by the ozone concentration adjuster 7,
is led into the second:stage bleacher 2. The gas discharged
out from the second stage bleacher after bleaching therein
is still:loaded with about 7 mg/Q of ozone, so this gas:is
further led into the third stage bleacher 3. After
completion o bleaching in this third stage bleacher,~:the
discharged gas has ozone concentration of 4 to 6 mg/Q,
so this gas is led into the ozone concentration adjuster 6
and mixed therein with a high concentratlon ozonous gas
supplied rom the ozone generator 5 to form an oZonous gas
with ozone concentration of 8 to lO mg/Q, and this gas
is introduced into the first stage bleacher l. This
ozonous gas performs a high-degree reaction with un~
bleached pulp, so that the gas discharged therefrom is
almost free of ozone. In this process, a part of the
discharge gaS from the thi.rd stage bleacher 3 may be led
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into the ozone concentration adjuster 7~ The same effect
as mentioned above is obtained in this case, too.
Fig. 2 shows another form of the pulp bleaching
system according to this invention. This system is same
as that of Fig. 1 in arrangement of the washers and bleachers.
The only difference is that the ozonous gas can be introduced
lndependently into the second~and third stage bleachers 12
and 13 through the ozone concentration adjusters 14 and
15, respectively. ~ ~
In this system, if an ozonous gas wlth ozone
concentration of lO to 15 mg/Q is introduced into the second
stage bleacher 12 and circulated~while using same un-
bleached~pulp as~used~in~Fig. 1, the gdS discharged ~rom
the second stage bleacher 12 has ozone concentration of
about 7 mg/Q, and when an ozonous gas with ozone concentra-
tion of 5 mg/Q is introduced into the third stage bleacher
13 and circulated, the gas discharged from this third
stage bleacher 13 has ozone concentration of 3.6 mg/Q.
In this system, a part of both circulation gases is guided
into an ozone concentra~ion adjuster 18 via by-pass pipes
16 and 17 and mixed with the high-concentration ozonous
gas from the ozone generator 19 to form an ozonous gas with
ozone concentration of 10 to 15 mg/Q, and this gas is
reacted with unbleached pulp. After the reaction with
unbleached pulp, the discharge gas has no or only a very
low ozone concentration (O ~ 0,75 mg/Q), indicating almost
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perfect consumption of ozone.
Thu.s, according to this paxallel introduct~on
and circulation system, bypasses 16 and 17 are provided
in connection to the second and third s-tage bleachers 12
and 13, and only that portion of introduced gas which was
increased by concentration adjustment is led into the
ozone concentration adjuster 18 for the first stage
bleacher 11 through the bypasses while any shortage is
supplied by the fresh ozonous gas from the ozone generator
19, and the thus prepared ozonous gas with the regulated
ozone concentration is supplied into the first stage
bleacher 11.
It has been ound, as noted from the afore-
shown Table 1, too r that ln the process from the second
stage bleaching step onward, thebrightnessincreasing
effect is not much influenced even if the ozone concentra-
tion is decreased gradually.
Thus, according to the multi-stage pulp bleaching
pro~ess of this invention, the gas discharged from the
second and succeeding stages in the bleaching process
is guided back into the first stage bleaching step where
the ozone reaction efficiency is high, 60 as -to reduce
the ozone consumption to close to zero to thereby realize
most effective utilization of expensive ozone. The
process of this invention is also advantageous from the
viewpoint of prevention of atmospheric pollution as no
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ozone-loaded gas is released into the atmosphere. This
invention is thus of extremely high industrial utility.
The process of this invention is described in
further detail herebelow by way of Examples.
Example 1
_
Unbleached hardwood kraft pulp (brightness
30.6%, re]ative viscosity 14.1~ was immersed in water
(at 21C) adjusted to pH 3.5 with sulphric acid and then
dehydrated to 300% water`content (300 parts of water per
100 parts of dry pulp) by using a dehydrator. Then,
the wet pulp thus obtained was put into a closed chamber
after fluffing and an ozonous gas with ozone concentratlon
of 10 mg/Q was supplied into the closed chamber for 10
minutes at a rate of 1 Q/min. The ozone concentration in
:
the exhaust gas discharged from the chamber was average
0.15 mg/Q for 10 minutes and the percentage of ozone-
consumption was 98.5%.
Then, the pulp thus treated with ozone as
mentioned above was extracted and washed at room
temperature with an aqueous NaOH solution containing
NaOH in an amount corresponding to 1 ~ by weight o the
pulp, again immersed in water adjusted to pH 3.5 with
sulphric acid, and then dehydrated to 300% water content
by using a dehydrator. Subsequently, the wet pulp obtained
was, as stated above, put into a closed chamber after
fluffing and an ozonous gas with ozone concentration of
-- 19 --
~3~ g
15 mg/Q was fed lnto the closed chamber. After the treat-
ment of 10 minutes, all the exhaust gas discharged from
the chamber was collected in a bag made of polyester
film. The ozone concentration in the exhaust gas was
7.8 mg/Q and the percentage of ozone-consumption was
48.0 %.
The exhaust gas obtained by bleaching the pulp
in the first step with an ozonous gas containing ozone
in a concentration of 10 mg/Q and after the extraction
with alkali solution treating~the pulp with an ozonous
gas containing ozone in a concentration of 15 mg/Q was
used in the second step as described above and thereafter
used as the introducing gas for the third step bleaching
the fluffed pulp ad~usted to pH 4.0 after the alkali-
extraction, washing and treatment with sulphric acid.
As the result of analyzing the exhaust gas discharged when
the pulp was bleached in the second step, it was found
that the ozone-concentration in the ozonous gas fed into
the second s*ep was about 8 mg/Q. The exhaust gas
discharged from the third bleaching step was 6.4 mg/Q.
The brightnessof bleached pulp obtained was 81.6 % and
the relative viscosity thereof was 6.3.
Further, when the first bleaching step was
carried out with the ozonous gas containing ozone in a
concentration of 10 mg/Q, the second bleaching step
with the ozonous gas containing ozone in a concentration
,
- 20 -
oE 15 mg/Q, the third bleaching step with the exhaust
gas discharged from the second bleaching step, and the
first bleaching step with the exhaust gas discharged
from the third bleaching step, it was found that the
exhaust gas discharged from all the bleaching steps
contained ozone in an amount of trace. Accordingly,
the ozone used was consumed substantially completely.
In this case, the whiteness of the pulp treated in the
first bleaching step was 36.5 ~ and the relative viscosity
thereof was 10.8.
~xample ~:
Unbleached hardwood Kraft pulp (brightness
30.6 ~, relative viscosity 14.1) was pre-treated by the
same manner as in Example 1 and bleached in the first
step with an ozonous gas containing ozone in a concentration
of 6 mg/Q. The pulp thus treated was bleached in the
second step with an ozonous gas containing ozone in a
concentration of 11.5 mg/Q, after the alkali-extraction,
washing and adjustment of pH. The resultant pulp was,
after the alkali-extraction, washing, and adjustment of
pH, bleached in the third step with an ozonous gas containing
ozone in a concentration of 15 mg/Q. In this case, the
ozone-concentration in the exhaust gas was 0.05 mg/Q at
the first bleaching step, 5.6 mg/Q at the second bleaching
step, and 11.3 mg/Q at the third bleaching step, respectively.
The first closed changer was stuffed with an
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unbleached pulp, the second closed chamber a pulp
treated for 10 minutes with an ozonous gas containing
ozone in a concentration of 6 mg/Q and subjected to
the alkali-extraction, washing, and adjustment of pH,
and the third closed chamber a pulp treated with an
ozonous gas containing ozone in à concentration of
6 mg/Q, subjected to the alkali-extraction, washing and
adjustment of pH, and followed by being treated with
an ozonous gas containing ozone in a concentration-o~
11.5 mg/Q and subjected to the alkali-extraction,
washing and adjustment o pH.~ Into the third chamber,
an ozonous gas containing ozone in a concentration~of
15 mg/Q was introduced. The exhaust gas d1scharged
from the third chamber was introduced into the second
chamber and the exhaust gas~discharged from the seoond
chamber was introduced into the first chamber. As the~
result of collecting and analyzing the exhaust gas dis-
charged from the first chamber, it was found that there
was ozone in an amount of trace. The brightness o~ pulp
obtained from the third chamber was 74.5~ and the relative
viscosity thereof was 7.4.
