EP2479342A1

EP2479342A1 - Ozone bleaching of pulp

Info

Publication number: EP2479342A1
Application number: EP12150702A
Authority: EP
Inventors: Torbjörn Reitberger
Original assignee: Wallenius Water AB
Current assignee: Wallenius Water AB
Priority date: 2011-01-12
Filing date: 2012-01-11
Publication date: 2012-07-25
Also published as: SE1150015A1

Abstract

The following invention relates to a method for bleaching of organic fibers comprising the steps of providing an aqueous suspension comprising organic fibers comprising lignin compounds, directing an ozone rich oxygen gas through said suspension for causing a substantial bleaching of said lignin compounds, providing carbon dioxide (CO 2 ) into said suspension, and reacting said carbon dioxide (CO 2 ) with ozonide radical anions (O3 ¢- ) produced as a result of said ozone bleaching and present in the suspension, for formation of oxygen (O 2 ) and carbonate radicals (CO 3 ¢- ) within the suspension.

Description

FIELD OF THE INVENTION

The present invention relates to a method for bleaching organic fibers comprising lignin structures.

BACKGROUND INFORMATION

The aim of chemical pulping of lignocelluloses, e.g., wood, is to obtain a fibrous cellulose-rich pulp by removing the lignin part. Industrially, this is done in sequences usually starting with nucleophilic chemistries in cooking liquor. During cooking lignin is broken down into smaller soluble units but the produced pulp acquires a dark colour from modified lignin residues. Further removal of these lignin residues requires complementary electrophilic chemistries allowing both delignification and bleaching.
The art of delignification and bleaching is to find an optimal balance between efficiency, i.e. lignin removal, and selectivity, i.e. cellulose conservation, using only environmentally acceptable processes. On account of the last criterion, use of chlorine gas for pulp delignification and bleaching is now abandoned in most industrial countries albeit that it is very efficient and selective. The reason for this is the formation of chlorinated organic compunds that severely harm the aquatic environment.
It is the common technique today to reduce the residual lignin content of the pulp obtained after a cooking stage by an oxygen treatment. This oxygen delignification further reduces the lignin content by oxidizing the lignin into smaller fragments which are removed from the fibers in one or several washing steps. The use of oxygen is very attractive from economic and environmental points of view but the chemistry involved results in formation of reduced oxygen species, e.g. hydroxyl radicals ^•OH, that through hydrogen atom abstraction can destroy cellulose molecules. This limited selectivity leads to impaired fiber strength properties.
If a white lignin free pulp is required, more selective chemistry must be adopted than that offered by oxygen delignification only. Since the oxidation potential of lignin is much lower than that of cellulose this can be achieved by appropriate one-electron oxidants. In practice, chlorine dioxide, ClO₂, has proven to be very useful for this purpose and production of high quality pulp is today achieved by using oxygen and chlorine dioxide in different bleaching sequences, possibly also including a final treatment with hydrogen peroxide to eliminate remaining chromophores.
The present technology of delignification and pulp bleaching is very efficient and fairly selective. From an environmental point of view, however, the process is not optimal since the use of chlorine dioxide leads to some formation of chlorinated reaction products, i.e. the problems associated with chlorine bleaching are reduced but not eliminated. For this reason, there is still a need to find more environmentally friendly bleaching chemicals in accordance with a general quest for best technology.
Ozone, O₃, is a powerful oxidant capable of reacting with most organic substances. This has inspired attempts to use ozone as a chlorine-free bleaching alternative that is superior from an environmental point of view. However, because of the difficulties of mastering radical formation, O₃-bleaching has not been widely adopted by the pulp industry.
The formation of radicals when ozone reacts with phenolic and non-phenolic lignin model compounds in aqueous solutions at different pH has been studied by M. Ragnar, et al. 1999, "Radical formation in ozone reactions with lignin and carbohydrate model compounds", Holzforschung 53(4), 292-298 and "A new mechanism in the ozone reaction with lignin-like structures", Holzforschung 53(4), 423-428. It was concluded that, in those systems, superoxide is the initial radical formed. Extremely reactive hydroxyl radicals are then generated predominantly by the reaction of superoxide with ozone.
Superoxide is the base form of the hydroperoxyl radical $HO$ $_{2}^{•} (pKa = 4.8) .$
In contrast to superoxide, the hydroperoxyl radical is a stronger one-electron oxidant than ozone so under sufficient acidic conditions generation of unselective hydroxyl radicals according to the above reaction scheme is not possible. Industrially, it is also an established fact that ozone bleaching requires acidic conditions, typically about pH 3. However, the hydroperoxyl radical is a precursor to hydrogen peroxide and the accumulation of this compound leads to formation of hydroxyl radicals in transition metal ion catalyzed reactions, which are difficult to block under acidic conditions, e.g.:

H⁺ + H₂O₂ + Fe²⁺ → Fe³⁺ + H₂O + ^•OH
Nemes et al. ("Kinetics and Mechanism of the Carbonate ion Inhibited Aqueous Ozone Decomposition" J.Phys. Chem. A 2000, 104(34), 7995-8000) have found that ozone is fairly stable in alkaline bicarbonate solutions. One possible explanation for the unexpected stability of O₃ in bicarbonate solutions could be that carbonate radical anions, ${CO}_{3}^{• -},$
, are formed, terminating the radical chain reactions normally leading to rapid ozone degradation by removal of the dominant chain carrier radicals ^•OH, $O_{2}^{• -}$
and $O_{3}^{• -} .$
It is stated that the carbonate radical anion can be produced from hydroxyl radicals in the presence of bicarbonate:
Like chlorine dioxide, the carbonate radical is a one-electron oxidant. The delignifying properties of this environmental benign reagent were investigated by David Stenman in his thesis "Advanced oxidation technologies for the pulp industry. - An investigation on the delignifying properties of the carbonate radical anion", Royal Institute of Technology, Stockholm, 2004, ISBN 91-7283-893-0.
The reactivity of the carbonate radical towards various lignin and carbohydrate model compounds was studied by pulse radiolysis. These studies showed that the carbonate radical anion oxidizes both phenolic and non-phenolic lignin structures very efficiently by one-electron transfer forming corresponding radical cations. The carbonate radical also reacts with carbohydrates but at a much lower rate. The kinetic selectivity of the carbonate radical should thus be sufficient for selective and efficient delignification of pulp.
Inspired by the promising results obtained on model compounds, Stenman performed carbonate radical bleaching studies on pulps and cotton linter suspensions. In these studies the carbonate radical was produced from peroxynitrite and carbon dioxide (S.V. Lymar and J.K. Hurst "Rapid Reaction between Peroxynitrite Ion and Carbon Dioxide" J.Am. Chem. Soc. 1995, 117, 8867-8858)
Peroxynitrite may also be used to generate hydroxyl radicals (G. Merényi et al. "Peroxynitrous Acid Homolysis into OH and NO2 Radicals" Chem Res. Toxicol. 1998,11, 712-713)

^-OONO + H⁺ → HOONO → [HO^• + NO₂]_cage
Peroxynitrite can thus be used in connection with pulp as a controlled source of hydroxyl and carbonate radicals. The studies on pulps and cotton linter suspensions revealed that:

The selectivity of the carbonate radical is much higher than that of the hydroxyl radical.
Radical cation formation, given by the carbonate radical, results in a rapid direct delignification and high brightness increase. This reflects that aromatic radical cation formation in the lignin is an efficient way to induce lignin fragmentation and bleaching. The hydroxyl radical on the other hand, causes mostly lignin modifications and far less fragmentations and bleaching.
Both radicals could degrade cotton linters as shown by viscosity and GP-SEC measurements. For the carbonate radical, >90 % of the viscosity losses could be recovered with a NaBH₄ treatment before the viscosity measurements whereas the recovery of viscosity after hydroxyl radical degradation and a subsequent NaBH₄ treatment was only about 40 %. This demonstrates that hydroxyl radicals abstract H-atoms randomly creating a statistical mixture of carbon centred radicals, some of these lead to direct cleavage of glucosidic bonds. In contrast, carbonate radicals mainly abstract H-atoms adjacent to hydroxyl groups, i.e. at C₂, C₃ and C₆. This intramolecular selectivity may reflect a polar effect, whereby hydrogen-atom abstractions from these positions are favoured by the less reactive carbonate radical. An abstraction at C₆ would also be sterically and statistically favoured.

From this survey it can be concluded that the carbonate radical, ${CO}_{3}^{• -},$
is a green eco-efficient oxidant that may substitute chlorine dioxide to achieve totally chlorine free (TCF) delignification. However, peroxynitrite, the precursor of the carbonate radical, ${CO}_{3}^{• -},$
used in the reported studies, is not a "green" oxidant, i.e. it is not environmentally acceptable, which is obviously a disadvantage. Therefore alternative ways of achieving bleaching of pulp with the carbonate radical would be desired.

OBJECTS OF THE INVENTION

It is an object of the present invention to overcome or at least minimize at least one of the drawbacks and disadvantages of the above described problems and difficulties associated with ozone bleaching, where harmful hydroxyl radicals are formed.
It is also an object of the present invention to provide an environmentally and industrially acceptable method for the generation of carbonate radicals, ${CO}_{3}^{• -} .$
Other objects of the present invention will become clear from the following description.

SUMMARY OF THE INVENTION

The objects of the invention are achieved by a modified ozone bleaching process which eliminates most of the problems associated with ozone bleaching, e.g. the difficulties of mastering radical formation. This improved ozone bleaching method provides both efficient lignin removal and sufficient cellulose conservation in order to enable industrially applicable ozone bleaching as an alternative to present bleaching processes.
The objects of the invention are achieved by method for bleaching of organic fibers comprising the steps of providing an aqueous suspension comprising organic fibers, said fibers comprising lignin compounds, directing an ozone rich oxygen gas through said suspension for causing a substantial bleaching and/or decomposition of said lignin compounds, providing carbon dioxide (CO₂) into said suspension thereby reacting said carbon dioxide (CO₂) with ozonide radical anions (O₃ ^•-) produced as a result of said ozone bleaching and present in the suspension, for formation of oxygen (O₂) and carbonate radicals (CO₃ ^•-) within the suspension.
It has unexpectedly been found that when carbon dioxide is present during the ozone bleaching step the first reduction reaction product formed, i.e. the ozonide radical anion, preferably reacts with carbon dioxide instead of reacting with the protons under proper conditions. Reaction with protons would yield unselective hydroxyl radicals that risk destroying cellulose molecules. Hereby, in a method according to the invention formation of the highly undesired unselective hydroxyl radicals is blocked during the process, and all the negative effects of this compound are substantially eliminated.
The method according to the invention therefore provides the opportunity of using the powerful oxidant ozone, O₃, in chlorine-free bleaching processes while at the same time being able to master the radical formation, which heretofore has been very difficult and represented an obstacle preventing implementation of O₃ bleaching within pulp industry. Thanks to the invention there is achieved an efficient method for bleaching and/or delignification which is also environmentally very favorable and which significantly improves the selectivity of the ozone bleaching as well as the efficiency thereof. This means the combination of the environmentally acceptable ozone and carbon dioxide in bleaching leads to eco-efficient process possible under very mild conditions without negative environmental impact. Chlorine dioxide as bleaching agent may hereby be substituted by ozone so as to achieve totally chlorine free (TCF) delignification.
Another advantage is that the components of the system are inexpensive and abundant which contributes to that an ozone/carbon dioxide stage according to the invention can be easily integrated with existing oxygen and hydrogen peroxide stages.
Thanks to the inventive bleaching method ozone may be used for bleaching of organic fibers with much reduced risk of formation of unselective hydroxyl radicals (OH), which would damage the cellulose and hemicelluloses chains of the pulp.
At the same time the method according to the invention leads to formation of carbonate radicals (CO₃ ^•-), which have proven to work very well for bleaching lignin compounds, including both phenolic and non-phenolic lignin structures.
According to one aspect of the invention the pH in said aqueous suspension is between 5 - 11, preferably between 7 - 10. This leads to the advantage that no delignification and bleaching stages require acid conditions, i.e. the pH does not have to be altered between the different bleaching stages in the bleaching sequences. Sharp pH or temperature gradients may thereby be substantially avoided. This allows more easy integration with alkaline oxygen and peroxide bleaching stages in the bleaching sequences.
Another aspect is the fact that in the suggested pH-range addition of metal ion chelating agents may be used to neutralize the formation of hydroxyl radicals by transition metal ion catalyzed decomposition of accumulated hydrogen peroxide. This means that said chelating agents may work well also when added to neutral or basic ozone bleaching steps comprising carbon dioxide. The harmful transition metal ions may thereby be captured by the chelating agents.
Other aspects and advantages will become clear in connection to the detailed description of the preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in the following in closer details by means of various embodiments thereof with reference to the accompanying drawing wherein identical numeral references have been used in the figure to denote corresponding components.

Fig. 1: illustrates schematically an example of the material and water flows in a pulp and paper production, in which an ozone bleaching step is incorporated in the process.
Fig. 2: illustrates schematically in more detail the bleaching process shown in Fig. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following detailed description, and the examples contained therein, are provided for the purpose of describing and illustrating certain embodiments of the invention only and are not intended to limit the scope of the invention in any way.
Fig. 1 shows a schematic drawing of exemplary material and water flows in pulp and paper/paperboard production. Raw materials 1 and water 2 are supplied to the production process 5. The product (e.g. paper, paperboard) 3 is produced and water vapour and polluted output process water 4 leaves the production process 5. Looking closer into the production process 5 it can be seen that the raw materials 1 and water 2 are supplied to a pulp and stock preparer 10 wherein an aqueous suspension comprising organic fibers comprising lignin compounds is formed. The aqueous pulp suspension 11 resulting from the pulp and stock preparer 10 is thereafter exposed to delignification and/or bleaching in a bleaching plant 12, which will later be described in more detail. The bleached pulp suspension 13 is introduced into forming equipment 14 producing different paper or paperboard products 3. Alternatively, the bleached pulp suspension 13 3 may be dewatered and dried thereby forming market pulp to be further processed.
Fig. 2 shows schematically a more detailed view of the bleaching plant 12 shown in Fig 1. A suspension of unbleached pulp 11 comprising fibers further comprising cellulose, hemicelluloses and lignin compounds is supplied to the bleaching plant 12. Conventionally, unbleached pulp 11 may first be treated with oxygen gas in a first oxygen delignification step 120 by adding appropriate chemicals 17, i.e. oxygen gas and other chemicals, and possibly water/process water 18 in order to obtain the proper concentration of pulp in the suspension 11. The oxygen delignified pulp 15 is then passed to a next bleaching step which in Fig. 2 is a carbon dioxide-reinforced ozone bleaching step 121 in accordance with the invention. Ozone gas and carbon dioxide and other chemicals, that possibly may be needed, are added to the carbon dioxide-reinforced ozone bleaching step 121 as illustrated by arrow 20. Water and/or process water 21 may be added so as to get a suitable concentration of pulp in the suspension to be ozone bleached. The pH of the suspension is adjusted so as to reach the proper pH-level. This may be done by addition of any conventional pH-adjusting medium, e.g. NaOH, H₂SO₄. Ozone bleached pulp 16 leaves the carbon dioxide-reinforced ozone bleaching step 121 and may either be of appropriate/final brightness or be passed to one or several additional bleaching steps 122 which may comprise one or several hydrogen peroxide bleaching step/s or any other conventional bleaching step in order to achieve the desired brightness of the bleached pulp. Finally, a suspension of bleached pulp 13 leaves the bleaching plant 12 and may be further treated, e.g. dewatered and dried if market pulp is to be produced or be treated and passed to a paper machine for production of paper, paperboard or other articles. Arrows 19, 22 and 25 indicates outlet streams of bleaching process water from the different bleaching steps 120, 121 and 122. Said streams may be handled in accordance with conventional handling and treatment within the pulp mills.
The ozone bleaching step 121 according to the invention will now be described in more detail.
In a first embodiment according to the invention the pulp suspension 15 enters the carbon dioxide-reinforced ozone bleaching step 121. A stream of carbon dioxide is fed in to the pulp in the step 121, which step may be carried out in a bleaching tower. Said carbon dioxide is thoroughly mixed into the pulp suspension so as to get the appropriate pH, which suitably is between 5-11, preferably between 7-10. Preferably the pressure of the carbon dioxide is 1 bar. Addition of a further pH-adjusting medium may be needed, e.g. in the form of NaOH, H₂SO₄. Thereafter, a gas stream comprising ozone and oxygen is added to the pulp suspension comprising the carbon dioxide, and mixed into the pulp suspension. The addition and mixing of said stream comprising ozone and oxygen may be done in any manner known per se, as well as adjustments of parameters such as retention time in the bleaching step.
In another embodiment according to the invention a gas stream comprising a mixture of carbon dioxide, ozone and oxygen in appropriate amounts/relations is added to the bleaching step 121 and mixed into the pulp suspension.
In still another embodiment according to the invention two different gas-streams (not shown), one comprising carbon dioxide and one comprising ozone/oxygen gas enter the bleaching step 121 and said gas-streams are simultaneously mixed into the pulp suspension.
In yet another embodiment according to the invention said carbon dioxide is added as aqueous solution to the bleaching step 121, either before or simultaneously as the ozone/oxygen gas is being added.
In a further embodiment according to the invention said carbon dioxide may be added to the pulp suspension before the pulp suspension enters the bleaching step 121, e.g. directly after a washing step between the bleaching steps. The carbon dioxide may also be added to the pulp suspension directly after the entrance of the suspension into the bleaching tower and an appropriate retention time is kept in the bleaching tower so that the carbon dioxide is being well mixed in into the pulp before ozone/oxygen gas is added to a position downstream from the position where carbon dioxide is added.
It is understood that the bleaching plant 12 shown in Figs. 1 and 2 further comprises conventional equipments which are not to be described here, e.g. pumps, washing equipments for washing the pulp between the bleaching stages as well as other conventional equipment.
It is emphasized that the carbon dioxide-reinforced ozone bleaching step may be included in existing bleaching steps in the mills either by adding the step as an additional step somewhere in the bleaching sequence or by replacing an existing bleaching step, e.g. a chlorine dioxide bleaching step or another less environmentally friendly bleaching step. Since ozone is a very efficient bleaching chemical it may also be possible to add the carbon dioxide-reinforced ozone bleaching step to an already existing bleaching sequence in the mill and to remove not only one but two or more existing bleaching steps which may have lower bleaching efficiency as compared to the bleaching efficiency of ozone.
The carbon dioxide-reinforced ozone bleaching step may be added at any position in the bleaching sequences in the mills. A carbon dioxide-reinforced ozone bleaching step after a first oxygen delignification step may be preferred but in another embodiments it may be preferred to start with a carbon dioxide-reinforced ozone bleaching step or to finish with such a step. Which position is the preferred position may depend on the characteristics of the pulp to be bleached as well as the desired characteristics of product to be produced. As the carbon dioxide-reinforced ozone bleaching step is carried out at alkaline pH no sharp pH gradients may be necessary as a consequence of adding said bleaching, which leads to that integrating the step at various positions in the production chain (e.g. with alkaline oxygen and peroxide bleaching stages) may be performed rather easily.
The chemical mechanisms behind the much improved selectivity and efficiency respectively of said carbon dioxide-reinforced ozone bleaching step will now be described.
As described by Ragnar et al. (vide supra) the ozonide radical anion (O₃ ^.-) is a primary reduction product when ozone reacts with lignin like compounds. In a conventional ozone bleaching step the ozonide radical anions react with protons present in the pulp suspension, thereby forming harmful hydroxyl radical anions according to:
It has surprisingly been found that if carbon dioxide (CO₂) is present during an ozone bleaching step, the ozonide radical anion may preferably react with carbon dioxide instead of reacting with the protons, leading to scavenging of the ozonide radical anions, the precursor of unselective hydroxyl radicals, by carbon dioxide according to the following reaction:
The formation of carbonate radicals (CO₃ ^.-) from carbon dioxide is heretofore unknown chemistry, and the insight of reaction (2) leads to possibilities of new improved ways of performing ozone bleaching of organic fibres, e.g. cellulose fibers, cotton fibers. By means of providing carbon dioxide into the aqueous pulp suspension (11 subjected to said ozone bleaching step 121) a method is achieved where carbon dioxide blocks the formation of unselective hydroxyl radicals and the problems with unselective reactions is much reduced. Another advantageous aspect of the method is the formation of carbonate radicals, which attain the same lignin decomposing and bleaching effects of pulp 15 as when using chlorine dioxide.
The method for ozone bleaching 121 according to the invention comprises the steps of firstly providing an aqueous suspension 15 comprising organic fibers with lignin compounds, and directing an ozone rich gas through said suspension 15 for causing a substantial bleaching and/or delignification of said lignin compounds. In connection with supply of said ozone rich gas it is provided carbon dioxide into said pulp suspension 15 (as described in connection to Fig. 1 and Fig. 2), which leads to the above described reaction (2). Hereby is achieved an eco-friendly ozone bleaching method which by means of providing carbon dioxide into the aqueous pulp suspension 15 subjected to said ozone bleaching step 121 is improved with regards to efficiency (lignin removal) and selectivity (cellulose conservation), using only environmentally acceptable compounds.
The exothermic reaction (2) mimics the previously described reaction between peroxynitrite and carbon dioxide, i.e. it can be described as an oxyl radical anion, O^•-, transfer:
This reaction was discovered in experiments were ozone reacted with veratryl acohol. The radical cation of veratryl alcohol, which is formed in a one-electron transfer reaction with ${CO}_{3}^{• -},$
yields veratraldehyde as the only reaction product. In contrast, only about 3% veratraldehyde is formed when hydroxyl radicals react with veratryl alcohol. The predominant reaction in this case is addition to the aromatic ring. The analysis of the veratraldehyde yield suggests that the rate of the reaction between $O_{3}^{• -}$
and CO₂ is k₂ ≥ 2·10⁵M^-1s^-1
Competing with the desired reaction is an undesired reaction with protons yielding unselective hydroxyl radicals:
This reaction should be diffusion controlled and thus k₁ ≈ 5· 10¹⁰M^-1s^-1 (J. Staehelin and J. Hoigné "Decomposition of Ozone in Water in the Presence of Organic Solutes Acting as Promoters and Inhibitors of Radical Chain Reactions" Environ. Sci. Technol. 1985, 19, 1208-1213)
The fraction of O reacting with CO₂ by formation of ${CO}_{3}^{• -}$
(η) is determined by the kinetic competition between reactions (1) and (2): $η = \frac{k_{2} [{CO}_{2}]}{k_{2} [{CO}_{2}] + k_{1} [H^{+}]} = \frac{1}{1 + \frac{k_{1} [H^{+}]}{k_{2} [{CO}_{2}]}}$
In aqueous solution, the fraction $\frac{[H^{+}]}{[{CO}_{2}]}$
is governed by hydration and acid-base equilibria:
The acid-base equilibrium is: $K_{a} = \frac{[H^{+}] [{HCO}_{3}^{-}]}{[H_{2} {CO}_{3}^{-}]}$
The hydration equilibrium is shifted far to the left, i.e. much in favour of dissolved CO₂ over H₂CO₃ (Latimer & Hildebrant "Reference Book of Inorganic Chemistry 3rd Ed." Macmillan, p.285). Hence $K_{a} = \frac{[H^{+}] [{HCO}_{3}^{-}]}{[H_{2} {CO}_{3}^{-}]} \approx \frac{[H^{+}] [{HCO}_{3}^{-}]}{[H_{2} {CO}_{3}^{-}]}$

which gives: $\frac{[H^{+}]}{[{CO}_{2}]} \approx \frac{K_{a}}{[{HCO}_{3}^{-}]}$
The fraction of O reacting with CO₂ by formation of ${CO}_{3}^{• -}$
(η) can now be expressed as: $η \approx \frac{1}{1 + \frac{k_{1} K_{a}}{k_{2} [{HCO}_{3}^{-}]}}$
With k₁ ≈ 5·10¹⁰M^-1s^-1, k₂≈2·10⁵M^-1s^-1, and K_a = 4.3·10^-7 (loc cit) a graphical solution of this equation is shown below. This calculation should be regarded as conservative since it is likely that k₂ > 2·10⁵M^-1s^-1. For example with k₂=2·10⁶M^-1s^-1 a calculation gives η=0,98 at 0.5 M HCO₃ ^-.
According to this analysis, preferred conditions for the implementation of the invention restrict pH of the fibre suspension to between 7-10 in solutions containing bicarbonate at a concentration exceeding 0.2M.
A high concentration of carbonate also offers the additional advantage that a significant fraction of adventitious hydroxyl radicals are scavenged:
Carbonate ions may also form metal ion complexes that help to neutralize the formation of hydroxyl radicals by reductive cleavage of accumulated hydrogen peroxide. Addition of common metal ion chelating agents can also be applied to strengthen this effect.
As will be understood by those skilled in the present field of art, numerous changes and modifications may be made to the above described and other embodiments of the present invention, without departing from its scope as defined in the appended claims.
For example, the present invention is not restricted to a special type of pulp but all known types of fibrous pulp suspensions comprising lignin compounds, e.g. cotton linters, chemical pulps, mechanical pulps, chemi-thermomechanical pulps, hardwood pulps, softwood pulps, eucalyptus pulps etc, may be bleached/delignified by ozone in the presence of carbon dioxide.
The chemical pulps may be of all known types, e.g. sulfate pulps, organosolv pulps and different kinds of sulfite pulps.
It is understood that the obj ects of the present invention set forth above, among those made apparent by the detailed description, shall be interpreted as illustrative and not in a limiting sense. The term "bleaching" used in the application and its appended claims may denote not only bleaching but delignification as well. Within the scope of the following claims the set-up of various alterations of the present invention may be possible.
It is further understood that ozone treatment within other technical areas where suspensions comprising fibers or other organic material, e.g. waste water, are treated with ozone may be enhanced by addition of carbon dioxide in accordance with the present invention. It should be noted that the above described aspects may be the subject for its own protection, as such in a separate divisional application. Hence, it is foreseen that this aspect of the invention may require a protection by its own, e.g. since it may be applicable per se also in other concepts than that defined by the independent claim in this application.

Claims

Method for bleaching of organic fibers comprising the steps of:
a. providing an aqueous suspension comprising organic fibers comprising lignin compounds,

b. directing an ozone rich gas through said suspension for causing a substantial bleaching and/or delignification of said lignin compounds, and

c. providing carbon dioxide (CO₂) into said suspension.
Method according to claim 1, wherein the step of providing carbon dioxide (CO₂) into said suspension is performed before performing step b.
Method according to claim 1, wherein the steps b and c are carried out simultaneously.
Method according to claim 1, wherein the step of providing carbon dioxide (CO₂) into said suspension is performed after performing step b.
Method according to any of claims 1-4, wherein carbon dioxide is added to said suspension as an aqueous solution.
Method according to any of claims 1-4, wherein carbon dioxide is added as a gas into said suspension.
The method according to claim 1, wherein the pH in said aqueous suspension is between 5 - 11, preferably between 7 - 10.
The method according to claim 1, wherein the concentration of carbon dioxide [CO²] in relation to concentration of proton [H⁺] within the suspension is high, preferably [CO₂]/[H⁺] being at least 10⁵, preferably at least 10⁶ and more preferred at least 10⁷.
Use of carbon dioxide for ozone bleaching of organic fibers in an aqueous suspension, characterized in that the relation between concentration of carbon dioxide and proton respectively [CO₂]/[H⁺] in said aqueous solution being at least 10⁵, preferably at least 10⁶ and more preferred 10'.