CA1103409A - Method of bleaching chemical pulp in multiple stages - Google Patents
Method of bleaching chemical pulp in multiple stagesInfo
- Publication number
- CA1103409A CA1103409A CA306,198A CA306198A CA1103409A CA 1103409 A CA1103409 A CA 1103409A CA 306198 A CA306198 A CA 306198A CA 1103409 A CA1103409 A CA 1103409A
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- ozone
- bleaching
- pulp
- gas
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-
- D—TEXTILES; PAPER
- D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
- D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
- D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
- D21C9/10—Bleaching ; Apparatus therefor
- D21C9/147—Bleaching ; Apparatus therefor with oxygen or its allotropic modifications
- D21C9/153—Bleaching ; Apparatus therefor with oxygen or its allotropic modifications with ozone
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- Engineering & Computer Science (AREA)
- Wood Science & Technology (AREA)
- Paper (AREA)
Abstract
TITLE OF THE INVENTION:
METHOD OF BLEACHING CHEMICAL PULP IN MULTIPLE STAGES
ABSTRACT OF THE DISCLOSURE:
Disclosed herein is an improvement in a method of bleaching a chemical pulp in multiple stages, in which the steps of treating the pulp with an ozone-containing gas and then washing the thus treated pulp are repeated successively.
In this improved method, an ozone-containing gas released from the second stage of ozone-treatment and at least one stage of ozone-treatment following to the second stage is introduced into the first stage of ozone-treatment.
According to this improved method, a gas not substantially containing ozone is finally released from the stages of ozone-treatment and the degradation of the chemical pulp is prevented sufficiently.
METHOD OF BLEACHING CHEMICAL PULP IN MULTIPLE STAGES
ABSTRACT OF THE DISCLOSURE:
Disclosed herein is an improvement in a method of bleaching a chemical pulp in multiple stages, in which the steps of treating the pulp with an ozone-containing gas and then washing the thus treated pulp are repeated successively.
In this improved method, an ozone-containing gas released from the second stage of ozone-treatment and at least one stage of ozone-treatment following to the second stage is introduced into the first stage of ozone-treatment.
According to this improved method, a gas not substantially containing ozone is finally released from the stages of ozone-treatment and the degradation of the chemical pulp is prevented sufficiently.
Description
~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 ~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~
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 _ 3~
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 ~ 12 -~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 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 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.
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 ~3~v3~
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 3~
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 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 ~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 , 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 ~33~
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.
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 ~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~
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 _ 3~
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 ~ 12 -~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 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 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.
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 ~3~v3~
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 3~
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 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 ~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 , 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 ~33~
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.
Claims (7)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A method of bleaching a wet-state chemical pulp comprising successively bleaching and washing said chemical pulp through at least three cycles of bleaching and washing, each bleaching step being conducted at a pH of 2.0 to 6.0 and in the presence of a gas containing 1 to 15 mg/l ozone, wherein unspent ozone from at least two of the second and subsequent cycles is recycled.
2. The method of claim 1, wherein ozone from the exhaust gas of the second cycle is fed to a subsequent cycle, and ozone from the exhaust gas of said subsequent cycle is fed to the first cycle.
3. The method of claim 1, wherein ozone from the exhaust gas of the ultimate cycle is fed to the immediately preceding cycle countercurrent to the flow of said chemical pulp and ozone from the exhaust gas of said immediately preceding cycle is fed to the first cycle.
4. The method of claim 2, wherein the ozone concentra-tion in the exhaust gas discharged from the third cycle is less than the ozone concentration in the exhaust gas in said second cycle.
5. The Method of claim 2, wherein the ozone con-centration in the exhaust gas of the last cycle is less than the ozone concentration in the exhaust gas of the preceding cycle.
6. A method of bleaching a wet-state chemical pulp comprising successively bleaching and washing said chemical pulp through at least three cycles of bleaching and washing, each bleaching step being conducted at a pH of 2.0 to 6.0 and in the presence of a gas containing 1 to 15 mg/l ozone, wherein unspent ozone from the exhaust gas of the ultimate cycle is fed to the immediately preceding cycle countercurrent to the flow of said chemical pulp and ozone from the exhaust gas of said immediately preceding cycle is fed to the first cycle.
7. A method of bleaching a wet-state chemical pulp comprising successively bleaching and washing said chemical pulp through at least three cycles of bleaching and washing, each bleaching step being conducted at a pH of 2.0 to 6.0 and in the presence of a gas containing 1 to 15 mg/l ozone, wherein unspent ozone from at least two of said cycles is recycled and a part of said unspent ozone, after adjusting the ozone concentration thereof is fed to the first cycle.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP7668977A JPS5430902A (en) | 1977-06-27 | 1977-06-27 | Industrial multistage pulp bleaching method |
| JP76689/77 | 1977-06-27 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1103409A true CA1103409A (en) | 1981-06-23 |
Family
ID=13612417
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA306,198A Expired CA1103409A (en) | 1977-06-27 | 1978-06-26 | Method of bleaching chemical pulp in multiple stages |
Country Status (4)
| Country | Link |
|---|---|
| JP (1) | JPS5430902A (en) |
| CA (1) | CA1103409A (en) |
| FI (1) | FI70610C (en) |
| SE (1) | SE440372B (en) |
Cited By (15)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5164043A (en) * | 1990-05-17 | 1992-11-17 | Union Camp Patent Holding, Inc. | Environmentally improved process for bleaching lignocellulosic materials with ozone |
| US5164044A (en) * | 1990-05-17 | 1992-11-17 | Union Camp Patent Holding, Inc. | Environmentally improved process for bleaching lignocellulosic materials with ozone |
| US5174861A (en) * | 1990-10-26 | 1992-12-29 | Union Camp Patent Holdings, Inc. | Method of bleaching high consistency pulp with ozone |
| US5181989A (en) * | 1990-10-26 | 1993-01-26 | Union Camp Patent Holdings, Inc. | Reactor for bleaching high consistency pulp with ozone |
| US5188708A (en) * | 1989-02-15 | 1993-02-23 | Union Camp Patent Holding, Inc. | Process for high consistency oxygen delignification followed by ozone relignification |
| US5211811A (en) * | 1989-02-15 | 1993-05-18 | Union Camp Patent Holding, Inc. | Process for high consistency oxygen delignification of alkaline treated pulp followed by ozone delignification |
| US5409570A (en) * | 1989-02-15 | 1995-04-25 | Union Camp Patent Holding, Inc. | Process for ozone bleaching of oxygen delignified pulp while conveying the pulp through a reaction zone |
| US5441603A (en) * | 1990-05-17 | 1995-08-15 | Union Camp Patent Holding, Inc. | Method for chelation of pulp prior to ozone delignification |
| LT3392B (en) | 1990-02-07 | 1995-08-25 | Chemiefaser Lenzing Ag | Process for bleaching of cellulose fibre excluding the use of chlorine |
| US5451296A (en) * | 1991-05-24 | 1995-09-19 | Union Camp Patent Holding, Inc. | Two stage pulp bleaching reactor |
| US5472572A (en) * | 1990-10-26 | 1995-12-05 | Union Camp Patent Holding, Inc. | Reactor for bleaching high consistency pulp with ozone |
| US5520783A (en) * | 1990-10-26 | 1996-05-28 | Union Camp Patent Holding, Inc. | Apparatus for bleaching high consistency pulp with ozone |
| US5554259A (en) * | 1993-10-01 | 1996-09-10 | Union Camp Patent Holdings, Inc. | Reduction of salt scale precipitation by control of process stream Ph and salt concentration |
| US5944952A (en) * | 1995-07-26 | 1999-08-31 | Beloit Technologies, Inc. | Method for bleaching high consistency pulp with a gaseous bleaching reagent |
| US6579411B1 (en) * | 1989-05-10 | 2003-06-17 | Andritz Oy | Bleaching medium consistency pulp with ozone |
Families Citing this family (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| AT395180B (en) * | 1989-08-16 | 1992-10-12 | Andritz Ag Maschf | METHOD FOR CRUSHING MATERIALS AND SYSTEM FOR IMPLEMENTING IT |
| US5389201A (en) * | 1992-02-28 | 1995-02-14 | International Paper Company | Bleaching of kraft cellulosic pulp employing ozone and reduced consumption of chlorine containing bleaching agent |
| JP3121917B2 (en) * | 1992-07-08 | 2001-01-09 | 住友重機械工業株式会社 | Pulp bleaching method with ozone |
Family Cites Families (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS5126302A (en) * | 1974-08-22 | 1976-03-04 | Hitachi Shipbuilding Eng Co | Sansoganjugasunyoru seruroosubutsushitsuno shorihoho |
| JPS526363A (en) * | 1975-07-04 | 1977-01-18 | Nikkan Ind | Pipe bender |
-
1977
- 1977-06-27 JP JP7668977A patent/JPS5430902A/en active Pending
-
1978
- 1978-06-21 SE SE7807120A patent/SE440372B/en not_active IP Right Cessation
- 1978-06-26 FI FI782032A patent/FI70610C/en not_active IP Right Cessation
- 1978-06-26 CA CA306,198A patent/CA1103409A/en not_active Expired
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5211811A (en) * | 1989-02-15 | 1993-05-18 | Union Camp Patent Holding, Inc. | Process for high consistency oxygen delignification of alkaline treated pulp followed by ozone delignification |
| US5409570A (en) * | 1989-02-15 | 1995-04-25 | Union Camp Patent Holding, Inc. | Process for ozone bleaching of oxygen delignified pulp while conveying the pulp through a reaction zone |
| US5188708A (en) * | 1989-02-15 | 1993-02-23 | Union Camp Patent Holding, Inc. | Process for high consistency oxygen delignification followed by ozone relignification |
| US6579411B1 (en) * | 1989-05-10 | 2003-06-17 | Andritz Oy | Bleaching medium consistency pulp with ozone |
| LT3392B (en) | 1990-02-07 | 1995-08-25 | Chemiefaser Lenzing Ag | Process for bleaching of cellulose fibre excluding the use of chlorine |
| US5441603A (en) * | 1990-05-17 | 1995-08-15 | Union Camp Patent Holding, Inc. | Method for chelation of pulp prior to ozone delignification |
| US5296099A (en) * | 1990-05-17 | 1994-03-22 | Union Camp Holding, Inc. | Environmentally improved process for bleaching lignocellulosic materials with oxygen, ozone and chlorine dioxide |
| US5164043A (en) * | 1990-05-17 | 1992-11-17 | Union Camp Patent Holding, Inc. | Environmentally improved process for bleaching lignocellulosic materials with ozone |
| US5164044A (en) * | 1990-05-17 | 1992-11-17 | Union Camp Patent Holding, Inc. | Environmentally improved process for bleaching lignocellulosic materials with ozone |
| US5181989A (en) * | 1990-10-26 | 1993-01-26 | Union Camp Patent Holdings, Inc. | Reactor for bleaching high consistency pulp with ozone |
| US5174861A (en) * | 1990-10-26 | 1992-12-29 | Union Camp Patent Holdings, Inc. | Method of bleaching high consistency pulp with ozone |
| US5472572A (en) * | 1990-10-26 | 1995-12-05 | Union Camp Patent Holding, Inc. | Reactor for bleaching high consistency pulp with ozone |
| US5520783A (en) * | 1990-10-26 | 1996-05-28 | Union Camp Patent Holding, Inc. | Apparatus for bleaching high consistency pulp with ozone |
| US5863389A (en) * | 1990-10-26 | 1999-01-26 | Union Camp Patent Holding, Inc. | Pulp bleaching reactor for dispersing high consistency pulp into a gaseous bleaching agent containing ozone |
| US5451296A (en) * | 1991-05-24 | 1995-09-19 | Union Camp Patent Holding, Inc. | Two stage pulp bleaching reactor |
| US5554259A (en) * | 1993-10-01 | 1996-09-10 | Union Camp Patent Holdings, Inc. | Reduction of salt scale precipitation by control of process stream Ph and salt concentration |
| US5693184A (en) * | 1993-10-01 | 1997-12-02 | Union Camp Patent Holding, Inc. | Reduction of salt scale precipitation by control of process stream pH and salt concentration |
| US5944952A (en) * | 1995-07-26 | 1999-08-31 | Beloit Technologies, Inc. | Method for bleaching high consistency pulp with a gaseous bleaching reagent |
Also Published As
| Publication number | Publication date |
|---|---|
| SE7807120L (en) | 1978-12-28 |
| SE440372B (en) | 1985-07-29 |
| FI70610C (en) | 1986-09-24 |
| FI782032A (en) | 1978-12-28 |
| FI70610B (en) | 1986-06-06 |
| JPS5430902A (en) | 1979-03-07 |
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