CA2144825A1 - Method of bleaching cellulose pulp - Google Patents
Method of bleaching cellulose pulpInfo
- Publication number
- CA2144825A1 CA2144825A1 CA002144825A CA2144825A CA2144825A1 CA 2144825 A1 CA2144825 A1 CA 2144825A1 CA 002144825 A CA002144825 A CA 002144825A CA 2144825 A CA2144825 A CA 2144825A CA 2144825 A1 CA2144825 A1 CA 2144825A1
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- CA
- Canada
- Prior art keywords
- pulp
- stage
- ozone
- bleaching
- temperature
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
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Classifications
-
- 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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- Life Sciences & Earth Sciences (AREA)
- Engineering & Computer Science (AREA)
- Wood Science & Technology (AREA)
- Paper (AREA)
Abstract
A method of producing TCF-pulp using an ozone stage (Z) as part of the bleaching sequence does not require cooling of the pulp prior to the Z stage, and then subsequent heating of the pulp after the Z stage, since the Z stage is practiced at medium consistency and at about the same temperature as the other bleaching stages (i.e. between about 80-120°C). Chemical (or other)cellulose pulp is treated by the steps of: (a) effecting acidification, washing or pressing, oxygen delignification, peroxide bleaching, or chelation of chemical cellulose pulp at a temperature between about 80-120°C; (b) immediately after step (a), ozone bleaching the pulp at a consistency of about 8-20% at a stabilized pH of about 2-7 and at a temperature between just over 80°C and about 120°C (preferably about 90-100°C), for less than about one minute (typically less than 30 seconds); and (c) immediately after step (b) effecting acidification, washing or pressing, oxygen delignification, peroxide bleaching, or chelation of the pulp at a temperature between about 80-120°C. The pulp pH is preferably stabilized by holding the pulp in liquid at a pH between about 2-5 for at least five to ten seconds (preferably 20 seconds-100 minutes) prior to the addition of ozone to the pulp in the Z stage.
Description
~ ` 21q~825 METHOD OF BLEACHING CELLULOSE PULP
BACKGROUND AND SUMMARY OF THE INVENTION
It has long been recognized that the ozone bleaching of cellulose pulp, regardless of the consistency of the pulp during bleaching, should be practiced 5 at relatively low temperatures. For example in U.S. patent 5,296,099 it is stated that the reaction temperature at which ozone bleaching is conducted is an important controlling factor and that the maximum temperature of the pulp fiber at which the reaction should be conducted should not exceed the temperature at which excessive degradation of the cellulose occurs. That is 10 stated to be, for southern U.S. softwoods, a maximum of roughly 50-65C.
Following this general wisdom in the art during bleaching sequences the pulp is typically cooled before an ozone stage, and reheated after the ozone stage, requiring separate heating and cooling equipment, and having a high energy penalty.
U.S. patent 4,372,812 discloses ozone bleaching carried out at low consistency. The patent describes a multi-stage bleaching process which begins with oxygen delignification at a temperature of about 80-120C, and then practices peroxide bleaching at a temperature of about 20-90C. After peroxide bleaching ozone bleaching takes place at a temperature of 15-60C.
This patent suggests that by practicing the method described therein the heating and cooling losses are lower than otherwise expected if ozone bleaching is used since if one goes from oxygen delignification directly to ozone bleaching the pulp must be significantly cooled, and then the temperature raised again if peroxide bleaching follows. This is one of many recognitions in the art that the "necessary" cooling of the pulp from prior stages, and heating of the pulp before feeding to subsequent bleaching stages, is undesirable when ozone bleaching is used, however the art has previously been unable to avoid the problem because of the degradation of cellulose expected to occur at temperatures above about 60C.
U.S. patent 4,959,124 discloses low consistency ozone bleaching in different sequences, with the ozone stage invariably being carried out at about room temperature. U.S. patent 5,164,043 discloses high consistency ozone 214~8~5 bleaching and requires that the temperature be within the range of about 50-65C in order to prevent deterioration of softwood.
U.S. patent 5,346,588 states that in order for ozone bleaching to be effectively practiced for medium consistency pulp the temperature must be 5 between 15-80C, preferably between 40-70C. In all of the examples set forth in the '588 patent the ozone stage temperature is always preceded by cooling of the pulp below about 50C, and followed by heating of the pulp to a temperature of about 65C or more. Kamyr AB carried out tests pursuant to the general disclosure in the '588 patent at the patent owner's mill in 10 Austria, at temperatures of 30C and 50C, which tests are reported in 1992 in Paperi Ja Puu - Volume 74, No. 9. According to these tests higher temperatures weaken the lignin removal and selectivity of the pulp, that is at a higher temperature the viscosity of the bleached pulp was lower, and it was suggested that the temperature should be maintained very low when bleaching 15 with ozone, even a temperature as low as approximately 0C being advantageous.
In addition to the above teachings in the art, patent applications filed by companies having expertise in pulp bleaching, including EKA Nobel, Sunds Defibrator, and Kamyr AB in the 1990s universally provide that ozone 20 treatment temperatures should be practiced below about 50C. Those in the art believe that when ozone is above 50C it has poor stability, and is impractical, and that the higher temperature the more the ozone deteriorates the fibers and therefore the poorer the quality of the pulp produced.
Thus the general knowledge in the art that ozone bleaching should be 25 practiced at low temperature often results in a high energy penalty when ozone bleaching is utilized. There are many situations where it is desirable to bleachTotally Chlorine Free (TCF) pulp, and oxygen delignification and peroxide bleaching are typically utilized in a bleaching sequence along with at least oneozone stage to produce TCF-pulp. The heating and cooling losses that are 30 present, required by the ozone bleaching, add a significant energy penalty tothe bleaching process. The oxygen and peroxide stages are typically practiced between 80-100C, and if it is necessary to cool the pulp down to about 50C, and then heat it back up to 80-100C, to accommodate the ozone stage, an inordinate amount of energy is wasted.
21~4825 lt has been recognized according to the present invention that the adverse consequences recognized by the prior art for higher temperature bleaching with ozone can be avoided in a simple and effective manner, with resultant enormous energy savings since it is no longer required to cool and 5 then reheat the pulp in order to effect ozone bleaching. According to the present invention it has been recognized that when pulp at medium consistency, e.g. about 5-25%, preferably about 8-20%, and most preferably about 8-15%, is treated for treatment time of about a minute or less (typically less than 30 seconds and sometimes even less than 10 seconds), there are no 10 deleterious effects due to ozone bleaching at temperatures just above 80C to about 120C. Therefore it is preferred that the bleaching of medium consistency cellulose pulp (typically chemical pulp but also TMP and other types of pulp) be practiced at about 85-120C, most desirably at about 90-100C, according to the invention .
According to one aspect of the present invention a method of bleaching cellulose pulp at medium consistency is provided, comprising the following steps: (a) Bleaching the pulp with ozone at a consistency of about 5-25% and at a temperature between just above 80C and about 120C for a time period short enough to preclude significant pulp degradation; and (b) bleaching or 20 delignifying the pulp with at least one of oxygen and peroxide in another stage at a temperature between about 80-120C, prior to or subsequent to the ozone bleaching in step (a).
In the practice of the method described above, the treatment time of step (a) is preferably less than a minute, and more preferably less than about 25 30 seconds, and the temperature is preferably between about 90- 100 C, and the pulp preferably has a consistency of about 8-20%. Steps (a) and (b) may comprise all of the bleaching stages to which the pulp is subjected so that the method produces TCF-pulp.
It has also been recognized according to the present invention that it is 30 very important that the pH of the pulp be stabilized prior to the ozone bleaching stage. If acid is merely added to the pulp and not allowed to stabilize before ozone is introduced, very rapid ozone reactions occur causing the pulp fiber itself may not be at the desired pH of about 2-7 (preferably about 2-5), resulting in poor bleaching. For instance, it has been found out that 21~4825 ., acidification for removing heavy metals from pulp prior to peroxide bleaching, and a subsequent wash does not ensure stabilized pH of the pulp. Therefore it is necessary that the pulp be subjected to appropriate pH conditions for at least 5 seconds, preferably about 20 seconds-10 minutes or more, before 5 ozone is added.
According to another aspect of the present invention a method of bleaching cellulose pulp at medium consistency is provided, comprising the following steps: (a) Stabilizing the pH of the pulp between about 2-7 by holding the pulp in liquid at a pH between about 2-7 for at least about 5 10 seconds. (b) Immediately after step (a) [that is without pressing or washing]bleaching the pulp with ozone at a consistency of about 5-25% and at a temperature between just above 80C and about 1 20C. And (c) bleaching or delignifying the pulp with at least one of oxygen and peroxide in another stage at a temperature between about 80-1 20C, prior to the acidification in 15 step (a) or subsequent to ozone bleaching in step (b).
The term Upressing or washing" within the context of the present application signifies the recent trend in the art, whereby the pulp or the like washing in the old sense of the word, i.e. displacement washing, is often replaced by a pressing operation which increases consistency of the processed 20 material, followed by diluting the pressed material. This way is known to be more effective and is often preferred to regular displacement washing.
In the practice of the method described above, the treatment time of step (b) is preferably less than a minute, and more preferably less than about 30 seconds, and the temperature is preferably between about 90-1 00C, and 25 the pulp preferably has a consistency of about 8-20%. Steps (a), (b) and (c) may comprise all of the bleaching stages to which the pulp is subjected so that the method produces TCF pulp.
Under almost all circumstances step (b) includes at least one prior treatment stage immediately before the ozone stage of step (a), and at least 30 one subsequent stage immediately after the ozone stage of step (a). The pulp is passed, according to the invention, from the prior stage to the ozone stage without positively cooling the pulp to reduce its temperature significantly (line losses may result but no concerted effort need be made to cool the pulp, as is common in the prior art), and the pulp may be passed from the ozone stage to 214~825 the subsequent stage without positively heating the pulp to increase its temperature significantly, the prior and subsequent stages being practiced at approximately the same temperature as the ozone stage of step (a). There may or may not be washing between the prior stage and the ozone stage, and 5 between the ozone and the subsequent stage, but in any event the washing will be practiced also at approximately the same temperature. One of the following steps may be practiced either immediately prior to, immediately after,or both prior to and after, step (a): acidification (A), washing or pressing, oxygen delignification (O), peroxide bleaching (P), and chelation (Q).
According to another aspect of the present invention a method of bleaching chemical cellulose pulp is provided comprising the following steps:
(a) Effecting acidification, washing or pressing, oxygen delignification, peroxide bleaching, or chelation of chemical cellulose pulp at a temperature between about 80-120C. (b) Immediately after step (a), ozone bleaching the pulp at a consistency of about 8-20% at a stabilized pH of about 2-7 and at a temperature between just over 80C and about 120C, for less than one minute. And, (c) immediately after step (b) effecting acidification, washing or pressing, oxygen delignification, peroxide bleaching, or chelation of the pulp at a temperature between about 80-120C.
It is the primary object of the present invention to effectively utilize at least one ozone bleaching stage in a bleaching sequence for cellulose pulp without requiring positive significant cooling of the pulp prior to the ozone stage and heating of the pulp after the ozone stage. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a graphical representation plotting Kappa No. versus temperature in exemplary ozone bleaching according to the present invention;
FIGURE 2 is a view like that of FIGURE 1 only plotting brightness versus 30 temperature;
21~82S
_ -7--FIGURE 3 is a view like that of FIGURE 1 only plotting viscosity versus temperature; and FIGURES 4 and 5 are schematic representations of exemplary method steps that may be practiced according to the invention.
DETAILED DESCRIPTION OF THE DRAWINGS
FIGURES 1 through 3 illustrate three plots of temperature and other parameters during ozone bleaching of chemical cellulose pulp. In the tests run to obtain the plots of FIGURES 1 through 3 chemical cellulose pulp having a consistency of about 13.2% was utilized. The pulp was washed before ozone 10 treatment, and its Kappa No. immediately preceding the ozone treatment was 6.4, its viscosity was 765 ml/g, and its brightness was 81.8% ISO. The bleaching tests were run at several different temperatures between 20C and 1 20C. In each case the ozone dose was 4 kg 03 per ton of pulp. The Kappa No., brightness, and viscosity of the pulp were determined at each test 15 temperature, and the results plotted in FIGURES 1 through 3. The amount of ozone added was completely consumed, and in each case typically in less than a minute.
As seen in FIGURE 1, in the tests of medium consistency pulp according to the invention there was no perceptible change in Kappa No. that was 20 dependent on the temperature. As seen in FIGURE 2, the brightness of the pulp reduced only very slightly throughout the temperature range, the total brightness reduction between 20C and 1 20C being less than 1 ISO unit. As indicated in FIGURE 3 the viscosity of the pulp was maintained, practically speaking, substantially unchanged throughout the entire temperature range, 25 being slightly higher between 90-100C than at 20C.
It will be apparent from the test results illustrated in FIGURES 1 through 3 that the sensitivity of ozone bleaching results to temperature is minimal when ozone bleaching is practiced at medium consistency for relatively short periods of time, that is for time periods short enough to preclude significant 30 pulp degradation. Therefore it is possible to ozone bleach medium consistency pulp at a temperature of just above 80C to about 120C without the adverse affects predicted by the conventional wisdom in the art.
In view of the very slight decrease in brightness illustrated in FIGURE 2, according to the present invention it may be desirable to add slightly more ozone when practicing the present invention than would be added at conventional ozone bleaching temperatures of about 50C, however the cost of the small amount of additional ozone (which would increase the brightness) is insignificant compared to the cost savings that ensue by eliminating the needfor equipment for cooling the pulp prior to the ozone bleaching, eliminating theneed for equipment for heating the pulp after ozone bleaching, and the energy costs that result from such cyclical cooling and heating.
According to the present invention numerous bleaching sequences utilizing ozone are practical without conventional cooling or heating. In the bleaching sequences which follow, "Z" refers to an ozone stage according to the invention, "A" refers to an acidification stage in which the pulp is typically treated with acid at a pH of 2-5 (preferably 3-5) for a time period of at least 10 seconds (e.g. about 20 seconds-100 minutes), "Q" refers to a chelation stage in which the pulp is treated with a chelating agent such as EDTA or DTPA at pH of about 3-7 (preferably about 5-6) for about 10-60 minutes, "0" refers to oxygen delignification (an alkaline stage), and "P" refers to a peroxide stage, typically in which about 5-30 kg hydrogen peroxide (or equivalent other peroxide)/ton of pulp is used. The 0 and P stages are typically practiced at about 80-120C, preferably at 80-100C. The peroxide stage is preferably pressurized, and oxygen may be used in the peroxide stage, for example 0-20 kg/ton of pulp 2 in the P stage. In the pulp sequences below a "-" between the stages refers to washing and/or pressing between the stages, whereas if no "-" appears between adjacent letters, but rather they are enclosed by parentheses that means that there is no between stage washing or pressing.
A "/" between the stages indicates that they are alternative stages.
Thus according to the present invention the sequence 0-Z-P is particularly suitable, and is possible without any special heating or cooling stages. That is the pulp is discharged from oxygen delignification stage without the necessity for cooling it, and is washed also without the necessity for cooling. After ozonation in the ozone (Z) stage at a temperature of just 21~4825 g above 80C -- about 120C -- the pulp may be washed or pressed, again without heating being necessary, and fed to the P stage, without the necessity for positively heating the pulp. The Q and A stages are particularly desirable to remove heavy metals prior to a P stage. Other bleachings stages that are 5 eminently suitable according to the present invention are: O-P-Z-P, O-Z-P-Z, as well as sequences O-(ZQ)-P-(ZQ)-P, O-(ZQ)-P-(ZP), O-Q/A-P-Z-P, O-Q/A-P-(ZQ)-P, O-Q/A-ZP-Z, or O-A-Z-P.
FIGURE 4 schematically illustrates a number of alternative bleaching sequences possible according to the invention. Stage 10 is a P, A, Q, or O
10 stage followed by between stage washing or pressing 12 prior to the Z stage 14. There is no significant change in temperature between the stages 10 and 14, the pulp being at medium consistency (about 5-25%, preferably about 8-20%) during the Z stage, at a pH of about 2-7 (preferably 2-5), at a temperature of just above 80-120C (preferably about 90-100C), and the 15 ozone bleaching time typically being about a minute or less (e.g. 30 seconds or less). The pulp is then passed to the between stage wash or press 16, and then to another treatment stage 18 which may be a P, A, Q, or O stage, again there being no significant temperature change between the stages 14, 18, no positive heating or cooling being necessary for both the stages 14, 18 to be 20 practiced within a temperature range that achieves desirable results.
FIGURE 5 is a schematic illustration of another sequence including a first stage 20 which is a P or O stage, followed by an acidification stage 22, then an ozone stage 24, followed by a P/O stage 26. Typically between stage washing or pressing is provided between the stage 20 and the acidification 25 stage 22, and between the stages 24, 26, although between stage washing or pressing is not necessary between Z and P stages (e.g. (ZP)).
Because the ozone reactions are so rapid according to the invention, where medium consistency pulp is utilized at high temperature the proper pH
of the pulp prior to addition of the ozone must be ensured in order to have 30 good results. The pH is desirably between about 2-7 during a Z stage, preferably 2-5. If the pH is not within that range throughout the entire pulp fibers, ozone reaction does not occur. It has been found according to the present invention that if the pulp is acidified immediately before ozone addition while the surface of the fibers and the water surrounding them may be at the 1 o-proper pH (e.g. 7 or below), the fiber itself will remain neutral or alkaline.
Therefore it is necessary that the acidification be stabilized -- that is that there be sufficient time for the acid to penetrate the fibers so that the fibers themselves are at the appropriate pH (e.g. 2-7) -- before ozone is added. That 5 is the exemplary acidification stage 22 in FIGURE 5, which is immediately before a Z stage (no intervening wash or press stage), should maintain all fibers in contact with liquid at the proper pH for at least 5 or 10 seconds. Since the pulp fibers are not harmed by exposing them to acid for long periods of time, there is no difficulty to hold all the fibers in contact with the proper pH liquid 10 in the stage 22 for 20 seconds to ten minutes, or even 10-100 minutes.
Theoretically it has been estimated that the time for diffusion of sodium from a fiber to the surrounding liquid is about 5-20 seconds, therefore it is highly desirable to stabilize the fibers by adding acid to them of the appropriate pH at least 20 seconds before the ozone is added in the Z stage.
It will thus be seen that according to the present invention a method of bleaching cellulose pulp (particularly a chemical cellulose pulp, such as kraft pulp, although other cellulose pulps such as TMP may also be bleached in the same manner), that achieves the same desirable results as conventional ozone bleaching but does not have the energy penalty normally associated therewith, 20 is provided. While the invention has been herein shown and described in what is presently conceived to be the most practical and preferred embodiment thereof, it will be apparent to those of ordinary skill in the art that many modifications may be made thereof within the scope of the invention, which scope is to be accorded the broadest interpretation of the appended claims so 25 as to encompass all equivalent methods and processes.
BACKGROUND AND SUMMARY OF THE INVENTION
It has long been recognized that the ozone bleaching of cellulose pulp, regardless of the consistency of the pulp during bleaching, should be practiced 5 at relatively low temperatures. For example in U.S. patent 5,296,099 it is stated that the reaction temperature at which ozone bleaching is conducted is an important controlling factor and that the maximum temperature of the pulp fiber at which the reaction should be conducted should not exceed the temperature at which excessive degradation of the cellulose occurs. That is 10 stated to be, for southern U.S. softwoods, a maximum of roughly 50-65C.
Following this general wisdom in the art during bleaching sequences the pulp is typically cooled before an ozone stage, and reheated after the ozone stage, requiring separate heating and cooling equipment, and having a high energy penalty.
U.S. patent 4,372,812 discloses ozone bleaching carried out at low consistency. The patent describes a multi-stage bleaching process which begins with oxygen delignification at a temperature of about 80-120C, and then practices peroxide bleaching at a temperature of about 20-90C. After peroxide bleaching ozone bleaching takes place at a temperature of 15-60C.
This patent suggests that by practicing the method described therein the heating and cooling losses are lower than otherwise expected if ozone bleaching is used since if one goes from oxygen delignification directly to ozone bleaching the pulp must be significantly cooled, and then the temperature raised again if peroxide bleaching follows. This is one of many recognitions in the art that the "necessary" cooling of the pulp from prior stages, and heating of the pulp before feeding to subsequent bleaching stages, is undesirable when ozone bleaching is used, however the art has previously been unable to avoid the problem because of the degradation of cellulose expected to occur at temperatures above about 60C.
U.S. patent 4,959,124 discloses low consistency ozone bleaching in different sequences, with the ozone stage invariably being carried out at about room temperature. U.S. patent 5,164,043 discloses high consistency ozone 214~8~5 bleaching and requires that the temperature be within the range of about 50-65C in order to prevent deterioration of softwood.
U.S. patent 5,346,588 states that in order for ozone bleaching to be effectively practiced for medium consistency pulp the temperature must be 5 between 15-80C, preferably between 40-70C. In all of the examples set forth in the '588 patent the ozone stage temperature is always preceded by cooling of the pulp below about 50C, and followed by heating of the pulp to a temperature of about 65C or more. Kamyr AB carried out tests pursuant to the general disclosure in the '588 patent at the patent owner's mill in 10 Austria, at temperatures of 30C and 50C, which tests are reported in 1992 in Paperi Ja Puu - Volume 74, No. 9. According to these tests higher temperatures weaken the lignin removal and selectivity of the pulp, that is at a higher temperature the viscosity of the bleached pulp was lower, and it was suggested that the temperature should be maintained very low when bleaching 15 with ozone, even a temperature as low as approximately 0C being advantageous.
In addition to the above teachings in the art, patent applications filed by companies having expertise in pulp bleaching, including EKA Nobel, Sunds Defibrator, and Kamyr AB in the 1990s universally provide that ozone 20 treatment temperatures should be practiced below about 50C. Those in the art believe that when ozone is above 50C it has poor stability, and is impractical, and that the higher temperature the more the ozone deteriorates the fibers and therefore the poorer the quality of the pulp produced.
Thus the general knowledge in the art that ozone bleaching should be 25 practiced at low temperature often results in a high energy penalty when ozone bleaching is utilized. There are many situations where it is desirable to bleachTotally Chlorine Free (TCF) pulp, and oxygen delignification and peroxide bleaching are typically utilized in a bleaching sequence along with at least oneozone stage to produce TCF-pulp. The heating and cooling losses that are 30 present, required by the ozone bleaching, add a significant energy penalty tothe bleaching process. The oxygen and peroxide stages are typically practiced between 80-100C, and if it is necessary to cool the pulp down to about 50C, and then heat it back up to 80-100C, to accommodate the ozone stage, an inordinate amount of energy is wasted.
21~4825 lt has been recognized according to the present invention that the adverse consequences recognized by the prior art for higher temperature bleaching with ozone can be avoided in a simple and effective manner, with resultant enormous energy savings since it is no longer required to cool and 5 then reheat the pulp in order to effect ozone bleaching. According to the present invention it has been recognized that when pulp at medium consistency, e.g. about 5-25%, preferably about 8-20%, and most preferably about 8-15%, is treated for treatment time of about a minute or less (typically less than 30 seconds and sometimes even less than 10 seconds), there are no 10 deleterious effects due to ozone bleaching at temperatures just above 80C to about 120C. Therefore it is preferred that the bleaching of medium consistency cellulose pulp (typically chemical pulp but also TMP and other types of pulp) be practiced at about 85-120C, most desirably at about 90-100C, according to the invention .
According to one aspect of the present invention a method of bleaching cellulose pulp at medium consistency is provided, comprising the following steps: (a) Bleaching the pulp with ozone at a consistency of about 5-25% and at a temperature between just above 80C and about 120C for a time period short enough to preclude significant pulp degradation; and (b) bleaching or 20 delignifying the pulp with at least one of oxygen and peroxide in another stage at a temperature between about 80-120C, prior to or subsequent to the ozone bleaching in step (a).
In the practice of the method described above, the treatment time of step (a) is preferably less than a minute, and more preferably less than about 25 30 seconds, and the temperature is preferably between about 90- 100 C, and the pulp preferably has a consistency of about 8-20%. Steps (a) and (b) may comprise all of the bleaching stages to which the pulp is subjected so that the method produces TCF-pulp.
It has also been recognized according to the present invention that it is 30 very important that the pH of the pulp be stabilized prior to the ozone bleaching stage. If acid is merely added to the pulp and not allowed to stabilize before ozone is introduced, very rapid ozone reactions occur causing the pulp fiber itself may not be at the desired pH of about 2-7 (preferably about 2-5), resulting in poor bleaching. For instance, it has been found out that 21~4825 ., acidification for removing heavy metals from pulp prior to peroxide bleaching, and a subsequent wash does not ensure stabilized pH of the pulp. Therefore it is necessary that the pulp be subjected to appropriate pH conditions for at least 5 seconds, preferably about 20 seconds-10 minutes or more, before 5 ozone is added.
According to another aspect of the present invention a method of bleaching cellulose pulp at medium consistency is provided, comprising the following steps: (a) Stabilizing the pH of the pulp between about 2-7 by holding the pulp in liquid at a pH between about 2-7 for at least about 5 10 seconds. (b) Immediately after step (a) [that is without pressing or washing]bleaching the pulp with ozone at a consistency of about 5-25% and at a temperature between just above 80C and about 1 20C. And (c) bleaching or delignifying the pulp with at least one of oxygen and peroxide in another stage at a temperature between about 80-1 20C, prior to the acidification in 15 step (a) or subsequent to ozone bleaching in step (b).
The term Upressing or washing" within the context of the present application signifies the recent trend in the art, whereby the pulp or the like washing in the old sense of the word, i.e. displacement washing, is often replaced by a pressing operation which increases consistency of the processed 20 material, followed by diluting the pressed material. This way is known to be more effective and is often preferred to regular displacement washing.
In the practice of the method described above, the treatment time of step (b) is preferably less than a minute, and more preferably less than about 30 seconds, and the temperature is preferably between about 90-1 00C, and 25 the pulp preferably has a consistency of about 8-20%. Steps (a), (b) and (c) may comprise all of the bleaching stages to which the pulp is subjected so that the method produces TCF pulp.
Under almost all circumstances step (b) includes at least one prior treatment stage immediately before the ozone stage of step (a), and at least 30 one subsequent stage immediately after the ozone stage of step (a). The pulp is passed, according to the invention, from the prior stage to the ozone stage without positively cooling the pulp to reduce its temperature significantly (line losses may result but no concerted effort need be made to cool the pulp, as is common in the prior art), and the pulp may be passed from the ozone stage to 214~825 the subsequent stage without positively heating the pulp to increase its temperature significantly, the prior and subsequent stages being practiced at approximately the same temperature as the ozone stage of step (a). There may or may not be washing between the prior stage and the ozone stage, and 5 between the ozone and the subsequent stage, but in any event the washing will be practiced also at approximately the same temperature. One of the following steps may be practiced either immediately prior to, immediately after,or both prior to and after, step (a): acidification (A), washing or pressing, oxygen delignification (O), peroxide bleaching (P), and chelation (Q).
According to another aspect of the present invention a method of bleaching chemical cellulose pulp is provided comprising the following steps:
(a) Effecting acidification, washing or pressing, oxygen delignification, peroxide bleaching, or chelation of chemical cellulose pulp at a temperature between about 80-120C. (b) Immediately after step (a), ozone bleaching the pulp at a consistency of about 8-20% at a stabilized pH of about 2-7 and at a temperature between just over 80C and about 120C, for less than one minute. And, (c) immediately after step (b) effecting acidification, washing or pressing, oxygen delignification, peroxide bleaching, or chelation of the pulp at a temperature between about 80-120C.
It is the primary object of the present invention to effectively utilize at least one ozone bleaching stage in a bleaching sequence for cellulose pulp without requiring positive significant cooling of the pulp prior to the ozone stage and heating of the pulp after the ozone stage. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a graphical representation plotting Kappa No. versus temperature in exemplary ozone bleaching according to the present invention;
FIGURE 2 is a view like that of FIGURE 1 only plotting brightness versus 30 temperature;
21~82S
_ -7--FIGURE 3 is a view like that of FIGURE 1 only plotting viscosity versus temperature; and FIGURES 4 and 5 are schematic representations of exemplary method steps that may be practiced according to the invention.
DETAILED DESCRIPTION OF THE DRAWINGS
FIGURES 1 through 3 illustrate three plots of temperature and other parameters during ozone bleaching of chemical cellulose pulp. In the tests run to obtain the plots of FIGURES 1 through 3 chemical cellulose pulp having a consistency of about 13.2% was utilized. The pulp was washed before ozone 10 treatment, and its Kappa No. immediately preceding the ozone treatment was 6.4, its viscosity was 765 ml/g, and its brightness was 81.8% ISO. The bleaching tests were run at several different temperatures between 20C and 1 20C. In each case the ozone dose was 4 kg 03 per ton of pulp. The Kappa No., brightness, and viscosity of the pulp were determined at each test 15 temperature, and the results plotted in FIGURES 1 through 3. The amount of ozone added was completely consumed, and in each case typically in less than a minute.
As seen in FIGURE 1, in the tests of medium consistency pulp according to the invention there was no perceptible change in Kappa No. that was 20 dependent on the temperature. As seen in FIGURE 2, the brightness of the pulp reduced only very slightly throughout the temperature range, the total brightness reduction between 20C and 1 20C being less than 1 ISO unit. As indicated in FIGURE 3 the viscosity of the pulp was maintained, practically speaking, substantially unchanged throughout the entire temperature range, 25 being slightly higher between 90-100C than at 20C.
It will be apparent from the test results illustrated in FIGURES 1 through 3 that the sensitivity of ozone bleaching results to temperature is minimal when ozone bleaching is practiced at medium consistency for relatively short periods of time, that is for time periods short enough to preclude significant 30 pulp degradation. Therefore it is possible to ozone bleach medium consistency pulp at a temperature of just above 80C to about 120C without the adverse affects predicted by the conventional wisdom in the art.
In view of the very slight decrease in brightness illustrated in FIGURE 2, according to the present invention it may be desirable to add slightly more ozone when practicing the present invention than would be added at conventional ozone bleaching temperatures of about 50C, however the cost of the small amount of additional ozone (which would increase the brightness) is insignificant compared to the cost savings that ensue by eliminating the needfor equipment for cooling the pulp prior to the ozone bleaching, eliminating theneed for equipment for heating the pulp after ozone bleaching, and the energy costs that result from such cyclical cooling and heating.
According to the present invention numerous bleaching sequences utilizing ozone are practical without conventional cooling or heating. In the bleaching sequences which follow, "Z" refers to an ozone stage according to the invention, "A" refers to an acidification stage in which the pulp is typically treated with acid at a pH of 2-5 (preferably 3-5) for a time period of at least 10 seconds (e.g. about 20 seconds-100 minutes), "Q" refers to a chelation stage in which the pulp is treated with a chelating agent such as EDTA or DTPA at pH of about 3-7 (preferably about 5-6) for about 10-60 minutes, "0" refers to oxygen delignification (an alkaline stage), and "P" refers to a peroxide stage, typically in which about 5-30 kg hydrogen peroxide (or equivalent other peroxide)/ton of pulp is used. The 0 and P stages are typically practiced at about 80-120C, preferably at 80-100C. The peroxide stage is preferably pressurized, and oxygen may be used in the peroxide stage, for example 0-20 kg/ton of pulp 2 in the P stage. In the pulp sequences below a "-" between the stages refers to washing and/or pressing between the stages, whereas if no "-" appears between adjacent letters, but rather they are enclosed by parentheses that means that there is no between stage washing or pressing.
A "/" between the stages indicates that they are alternative stages.
Thus according to the present invention the sequence 0-Z-P is particularly suitable, and is possible without any special heating or cooling stages. That is the pulp is discharged from oxygen delignification stage without the necessity for cooling it, and is washed also without the necessity for cooling. After ozonation in the ozone (Z) stage at a temperature of just 21~4825 g above 80C -- about 120C -- the pulp may be washed or pressed, again without heating being necessary, and fed to the P stage, without the necessity for positively heating the pulp. The Q and A stages are particularly desirable to remove heavy metals prior to a P stage. Other bleachings stages that are 5 eminently suitable according to the present invention are: O-P-Z-P, O-Z-P-Z, as well as sequences O-(ZQ)-P-(ZQ)-P, O-(ZQ)-P-(ZP), O-Q/A-P-Z-P, O-Q/A-P-(ZQ)-P, O-Q/A-ZP-Z, or O-A-Z-P.
FIGURE 4 schematically illustrates a number of alternative bleaching sequences possible according to the invention. Stage 10 is a P, A, Q, or O
10 stage followed by between stage washing or pressing 12 prior to the Z stage 14. There is no significant change in temperature between the stages 10 and 14, the pulp being at medium consistency (about 5-25%, preferably about 8-20%) during the Z stage, at a pH of about 2-7 (preferably 2-5), at a temperature of just above 80-120C (preferably about 90-100C), and the 15 ozone bleaching time typically being about a minute or less (e.g. 30 seconds or less). The pulp is then passed to the between stage wash or press 16, and then to another treatment stage 18 which may be a P, A, Q, or O stage, again there being no significant temperature change between the stages 14, 18, no positive heating or cooling being necessary for both the stages 14, 18 to be 20 practiced within a temperature range that achieves desirable results.
FIGURE 5 is a schematic illustration of another sequence including a first stage 20 which is a P or O stage, followed by an acidification stage 22, then an ozone stage 24, followed by a P/O stage 26. Typically between stage washing or pressing is provided between the stage 20 and the acidification 25 stage 22, and between the stages 24, 26, although between stage washing or pressing is not necessary between Z and P stages (e.g. (ZP)).
Because the ozone reactions are so rapid according to the invention, where medium consistency pulp is utilized at high temperature the proper pH
of the pulp prior to addition of the ozone must be ensured in order to have 30 good results. The pH is desirably between about 2-7 during a Z stage, preferably 2-5. If the pH is not within that range throughout the entire pulp fibers, ozone reaction does not occur. It has been found according to the present invention that if the pulp is acidified immediately before ozone addition while the surface of the fibers and the water surrounding them may be at the 1 o-proper pH (e.g. 7 or below), the fiber itself will remain neutral or alkaline.
Therefore it is necessary that the acidification be stabilized -- that is that there be sufficient time for the acid to penetrate the fibers so that the fibers themselves are at the appropriate pH (e.g. 2-7) -- before ozone is added. That 5 is the exemplary acidification stage 22 in FIGURE 5, which is immediately before a Z stage (no intervening wash or press stage), should maintain all fibers in contact with liquid at the proper pH for at least 5 or 10 seconds. Since the pulp fibers are not harmed by exposing them to acid for long periods of time, there is no difficulty to hold all the fibers in contact with the proper pH liquid 10 in the stage 22 for 20 seconds to ten minutes, or even 10-100 minutes.
Theoretically it has been estimated that the time for diffusion of sodium from a fiber to the surrounding liquid is about 5-20 seconds, therefore it is highly desirable to stabilize the fibers by adding acid to them of the appropriate pH at least 20 seconds before the ozone is added in the Z stage.
It will thus be seen that according to the present invention a method of bleaching cellulose pulp (particularly a chemical cellulose pulp, such as kraft pulp, although other cellulose pulps such as TMP may also be bleached in the same manner), that achieves the same desirable results as conventional ozone bleaching but does not have the energy penalty normally associated therewith, 20 is provided. While the invention has been herein shown and described in what is presently conceived to be the most practical and preferred embodiment thereof, it will be apparent to those of ordinary skill in the art that many modifications may be made thereof within the scope of the invention, which scope is to be accorded the broadest interpretation of the appended claims so 25 as to encompass all equivalent methods and processes.
Claims (20)
1. A method of bleaching cellulose pulp at medium consistency, comprising the steps of:
(a) stabilizing the pH of the pulp between about 2-7 by holding the pulp in liquid at a pH between about 2-7 for at least five seconds;
(b) immediately after step (a), bleaching the pulp with ozone at a consistency of about 5-25% and at a temperature between just above 80°C
and about 120°C; and (c) bleaching or delignifying the pulp with at least one of oxygen and peroxide in another stage at a temperature between about 80-120°C, prior to step (a), or subsequent to the ozone bleaching in step (b).
(a) stabilizing the pH of the pulp between about 2-7 by holding the pulp in liquid at a pH between about 2-7 for at least five seconds;
(b) immediately after step (a), bleaching the pulp with ozone at a consistency of about 5-25% and at a temperature between just above 80°C
and about 120°C; and (c) bleaching or delignifying the pulp with at least one of oxygen and peroxide in another stage at a temperature between about 80-120°C, prior to step (a), or subsequent to the ozone bleaching in step (b).
2. A method as recited in claim 1 wherein step (a) is practiced by holding the pulp in liquid at a pH of 2-7 for at least about 20 seconds.
3. A method as recited in claim 2 wherein step (b) is practiced in a treatment time of less than about 30 seconds.
4. A method as recited in claim 1 wherein step (b) is practiced at a temperature between about 90-100°C.
5. A method as recited in claim 1 wherein step (a) is practiced to stabilize the pulp at a pH of about 2-5.
6. A method as recited in claim 1 further comprising washing or pressing after step (b).
7. A method as recited in claim 5 wherein the pulp during the practice of each of steps (a) and (b) has a consistency of about 8-20%.
8. A method as recited in claim 7 wherein step (c) includes at least one prior treatment stage immediately before the ozone stage of step (b), and at least one subsequent stage immediately after the ozone stage of step (b); and wherein the pulp is passed from the prior stage to the ozone stage without positively cooling the pulp to reduce its temperature significantly, and whereinthe pulp is passed from the ozone stage of step (b) to the subsequent stage without positively heating the pulp to increase its temperature significantly, the prior and subsequent stages being practiced at approximately the same temperature as the ozone stage of step (b).
9. A method as recited in claim 1 wherein step (c) includes at least one prior treatment stage immediately before the ozone stage of step (b), and at least one subsequent stage immediately after the ozone stage of step (b); and wherein the pulp is passed from the prior stage to the ozone stage without positively cooling the pulp to reduce its temperature significantly, and whereinthe pulp is passed from the ozone stage of step (b) to the subsequent stage without positively heating the pulp to increase its temperature significantly, the prior and subsequent stages being practiced at approximately the same temperature as the ozone stage of step (b).
10. A method as recited in claim 9 further comprising washing or pressing between at least one of the prior stage and the ozone stage, and the ozone stage and the subsequent stage.
11. A method as recited in claim 1 wherein step (b) is practiced so that there is a treatment time of less than about 30 seconds.
12. A method as recited in claim 7 wherein step (a) is practiced at a temperature between about 90-100°C.
13. A method as recited in claim 11 wherein step (a) is practiced to stabilize the pulp at a pH of about 2-5.
14. A method as recited in claim 1 wherein the pulp during the practice of step (b) has a consistency of about 8-20%.
15. A method as recited in claim 1 wherein the pulp is chemical cellulose pulp, and wherein steps (b) and (c) comprise all of the bleaching stages to which the pulp is subjected, so that the method produces TCF-pulp.
16. A method as recited in claim 1 wherein step (c) comprises a prior stage, and wherein the pulp is fed from the prior stage to the ozone stage of step (b) without positively cooling the pulp to significantly reduce the temperature thereof.
17. A method as recited in claim 1 wherein step (c) comprises a subsequent stage, and wherein the pulp is fed from the ozone stage of step (b) to the subsequent stage without positively heating the pulp to significantly increase the temperature thereof.
18. A method as recited in claim 1 wherein one of the following steps is practiced immediately after step (b): acidification, washing or pressing, oxygen delignification, peroxide bleaching, or chelation.
19. A method of bleaching chemical cellulose pulp comprising the steps of:
(a) effecting acidification, washing or pressing, oxygen delignification, peroxide bleaching, or chelation of chemical cellulose pulp at a temperature between about 80-120°C;
(b) stabilizing the pH of the pulp between about 2-7 by holding the pulp in liquid at a pH between about 2-7;
(c) immediately after step (b), ozone bleaching the pulp at a consistency of about 8-20% at a stabilized pH of about 2-7 and at a temperature between just over 80°C and about 120°C, for less than about one minute; and (d) immediately after step (c) effecting acidification, washing or pressing, oxygen delignification, peroxide bleaching, or chelation of the pulp at a temperature between about 80-120°C.
(a) effecting acidification, washing or pressing, oxygen delignification, peroxide bleaching, or chelation of chemical cellulose pulp at a temperature between about 80-120°C;
(b) stabilizing the pH of the pulp between about 2-7 by holding the pulp in liquid at a pH between about 2-7;
(c) immediately after step (b), ozone bleaching the pulp at a consistency of about 8-20% at a stabilized pH of about 2-7 and at a temperature between just over 80°C and about 120°C, for less than about one minute; and (d) immediately after step (c) effecting acidification, washing or pressing, oxygen delignification, peroxide bleaching, or chelation of the pulp at a temperature between about 80-120°C.
20. A method of bleaching cellulose pulp at medium consistency, comprising the steps of:
(a) bleaching the pulp with ozone at a consistency of about 5-25% and at a temperature between just above 80°C and about 120°C for a time period of less than about thirty seconds; and (b) bleaching or delignifying the pulp with at least one of oxygen and peroxide in another stage at a temperature between about 80-120°C, prior to or subsequent to the ozone bleaching in step (a).
(a) bleaching the pulp with ozone at a consistency of about 5-25% and at a temperature between just above 80°C and about 120°C for a time period of less than about thirty seconds; and (b) bleaching or delignifying the pulp with at least one of oxygen and peroxide in another stage at a temperature between about 80-120°C, prior to or subsequent to the ozone bleaching in step (a).
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| FI941229A FI98839B (en) | 1994-03-16 | 1994-03-16 | Method for bleaching of pulp |
| FI941229 | 1994-03-16 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2144825A1 true CA2144825A1 (en) | 1995-09-17 |
Family
ID=8540324
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002144825A Abandoned CA2144825A1 (en) | 1994-03-16 | 1995-03-16 | Method of bleaching cellulose pulp |
Country Status (3)
| Country | Link |
|---|---|
| CA (1) | CA2144825A1 (en) |
| FI (1) | FI98839B (en) |
| SE (1) | SE511743C3 (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN113897813A (en) * | 2021-09-17 | 2022-01-07 | 华南理工大学 | Method for bleaching eucalyptus board skin pulp by ozone in short sequence |
Families Citing this family (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FI105701B (en) | 1995-10-20 | 2000-09-29 | Ahlstrom Machinery Oy | Method and arrangement for treatment of pulp |
-
1994
- 1994-03-16 FI FI941229A patent/FI98839B/en active
-
1995
- 1995-03-16 CA CA002144825A patent/CA2144825A1/en not_active Abandoned
- 1995-03-16 SE SE9500926A patent/SE511743C3/en not_active IP Right Cessation
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN113897813A (en) * | 2021-09-17 | 2022-01-07 | 华南理工大学 | Method for bleaching eucalyptus board skin pulp by ozone in short sequence |
Also Published As
| Publication number | Publication date |
|---|---|
| SE511743C2 (en) | 1999-11-15 |
| FI98839B (en) | 1997-05-15 |
| SE9500926L (en) | 1995-09-17 |
| FI941229A0 (en) | 1994-03-16 |
| SE9500926D0 (en) | 1995-03-16 |
| FI941229A (en) | 1995-09-17 |
| SE511743C3 (en) | 2000-01-10 |
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