Final Pulp Bleaching by Ozonation: Chemical Justification and Practical Operating Conditions

By D. Lachenal, G. Pipon, and C. Chirat

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and was likely more related to colour removal than to delignification. Finally, if one admits that two different chemistries are required during the whole bleaching process, one may argue about the justi-fication of using one single reagent (ClO2) in the whole bleaching process.

Actually, very few basic investigations have been carried out so far to understand the chemis-try involved at the very end of the pulp bleaching process [2, 3].

One way to approach this understanding is to investigate the effect of the bleaching agents on the chemical structures likely to be present at the latest stage of the process. A lot of work has already been done on the mechanism of lignin degradation during chlorine dioxide or ozone stages [7, 8]. Ring opening and resulting intro-duction of carboxyl groups on the lignin moieties offer reasonable explanation for lignin removal. Even though the formation of new chromophores has been mentioned, like quinones in the case of chlorine dioxide [7, 9], no particular attention has been paid to the evolution of colour during bleach-ing nor to the chemistry of some coloured groups such as quinones.

In the first part, this paper will concentrate on the evolution of colour when chromophores models react with chlorine dioxide and ozone. In a second part it will be shown that ozone bleaching can be performed under conditions which improve its efficiency.

EXPERIMENTALPulp samplesTwo mixed hardwood kraft pulp samples (bright-

he colour of unbleached pulp is mainly due to chromophores which are formed on lignin during kraft cooking. Even though wood carbohydrates may in theory form coloured substances,

their presence in unbleached pulp has never been proved. Moreover, a recent study showed that such chromophores, if they exist, should be easily destroyed and should not survive the first bleach-ing stages [1]. It has been proposed that the last chromophores still present in the pulp at the end of the bleaching process would contain quinone groups possibly attached to phenolic residues [2, 3]. Considering the tiny quantities in which those chromophores must be, the reason why a relatively large excess of reagents must be applied to get rid of them is difficult to understand. Also, no jus-tification really exists as to the rationale of using chlorine dioxide at that position.

Bleaching sequences of the D (EO) DED type are today considered the reference. Because some pulps did not respond very well to this treatment it has been proposed to add some peroxide in the extraction stages. Very often this solution is more economical than increasing the chlorine dioxide charges [4]. More recently ozone was proposed to replace or complement the last D stage [5]. Spec-tacular results were observed, with ozone quantities as low as 0.05% being claimed to increase the final brightness by several units [6]. Applying ozone at this position was different from its previous uses. In particular an acidic pH was not required which represented a definite practical advantage. It also indicated that the chemistry of final bleaching was not the same as that prevailing in the first stages

T

Abstract: In an attempt to understand why, in pulp bleaching, the final brightness points are so difficult to gain despite the very low content of residual chromophores, several model compounds representing chemical structures which may still be present in semi-bleached pulp were submitted to various charges of chlorine dioxide and ozone. Depending on the model (quinones or lignin fragment) and on the reagent, different results were obtained. ClO2, when it reacted with the lignin fragment, generated new coloured chromophores which were resistant to further degradation. O3 also produced new coloured groups. However, the latter were entirely degraded with an excess of reagent. Moreover, the quinone models were very reluctant to degradation by ClO2. On the contrary they were easily destroyed by O3. These results may explain the pulp resistance to chlorine dioxide in final bleaching and the better efficiency of an ozone treatment for this purpose.

The conditions of an ozone stage performed at the end of a bleaching sequence were not as critical as when ozone served as delignifying agent. It was shown that the ozone stage could be advantageously conducted at high temperature (80°C).

C. ChiratGrenoble INP-PagoraSt. Martin d’Hères, France

G. PiPonGrenoble INP-PagoraSt. Martin d’Hères, France

D. LaChenaLGrenoble INP-PagoraSt. Martin d’Hères, France

WINNER OF THE I.H. WELDON

AWARD

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ide (D stage) and with ozone (Z stage). The results in Table I clearly show the superiority of the ozone over chlorine diox-ide in final bleaching. The reason for such a difference must be related to the nature of the chromophores still present in the pulp before final bleaching.

Even though those chromophores are not precisely known it seems reasonable to assume that they should contain phenolic and quinone groups belonging to residual lignin fragments [2, 3]. Three models were chosen to cover these possibilities: a commercial lignin (lignosulphonate was preferred in order to enhance the solubility in neutral conditions), para-benzoquinone, and naphtoquinone (sulphonated for the same reason). Solutions of the models in water were submitted to D and Z condi-tions and the colour of the solution was followed by measuring the absorbance at 457 nm or 427 nm. The results are given in Figures 1-3.

Figure 1 shows that the colour of the lignin solution first increased dur-ing the treatment. Then colour removal took place. It was interesting to notice that colour was not completely eliminated when ClO2 was used in large excess, con-trary to what happened with O3 which ultimately led to a colourless solution. In the case of ClO2 treatment, colour forma-tion may result from the formation of qui-nones as described in many studies dealing with the ClO2 oxidation of the phenolic units in lignin [7]. Quinones should not be formed during ozonation. However new carbonyl groups are created which may increase the light absorption of the existing chromophores. Further application of the reagents reduced the colour. Total destruc-tion of the original and new chromophores was possible with O3 only. Some of them resisted the ClO2 treatment.

Figure 2 confirms that quinone groups were rather resistant to ClO2 oxidation but were degraded by O3. Both Figures 1 and 2 clearly suggest that ozone would be more appropriate to destroy pulp chromophores, which is in accordance with the bleaching results in Table I.

6 mmol l-1 (except for naphthoquinone: 1 mmol l-1). These concentrations were chosen in order to be in the suitable range of absorbance of the spectrophotometer (Unicam UV500, Thermospectronic).

Ozone treatment1.5 mmol (0.25 mmol for naphthoqui-none) of the compounds were dissolved into 250 ml of deionized water. This solution was ozonated in a glass flask by injecting directly gaseous ozone at 20°C. This temperature is low enough to favour ozone solubility. The quantity of ozone that was introduced in the solution at a flow rate of 1 l min-1 was measured by an ozone analyzer (BMT 961). The residual gas was directed to a 400 ml trap solution of 20 g l-1 potassium iodide (Roth) where it was titrated by iodometry. Thus, the exact quantity of ozone consumed by the solutions of the model compounds could be calculated by difference.

Chlorine dioxide treatment1.2 mmol (0.2 mmol for naphthoquinone) of the compounds were dissolved into 100 ml of deionized water. Then, a given quantity of deionized water and of chlorine dioxide was added to reach 200 ml. This solution was maintained in a closed and opaque reactor at 70°C and under agitation during half an hour. Then, it was cooled in an ice bath till its temperature reached 20°C. As chlorine dioxide has a yellow colour which can interfere in the visible spectra, the residual chlorine dioxide was removed by a flow of nitrogen gas injected directly in the solution. The extracted chlorine dioxide was directed into a 350 ml trap solution of 20 g l-1 potassium iodide where it was titrated by iodometry.

The effects of ozone and chlorine diox-ide on the model compounds were fol-lowed by UV-Vis spectrophotometry at 457 nm. The advantage is that this wave-length is used for the measurement of the brightness of pulp and paper. An exception was para-benzoquinone for which the analysis was performed at 427 nm since the absorbance at 457 nm was very low. The pH was monitored in all cases.

RESULTS AND DISCUSSIONFinal bleaching with chlorine dioxide and ozone The mixed hardwood kraft pulp of 85.1% brightness was treated with chlorine diox-

ness 83.7% and 85.1% ISO) were provided by the Alizay mill in France. The pulp had been bleached in the mill by the D0 (EP) D1 sequence and sampled ahead of the final D2 stage.

Reagents and bleaching stagesOzone was produced in a laboratory ozone generator (LN 103, Ozonia) from pure oxygen at a concentration of 50-60 mg l-1. Chlorine dioxide was produced in the labo-ratory from the reaction between sulphuric acid and sodium chlorite at a concentration of 8 g l-1 in water. For practical reasons the Z stages were carried out at high consis-tency. The mill pulp was diluted in process water, then centrifuged to 35% consistency and fluffed. The ozone treatment was car-ried out in a rotating spherical glass reactor at the desired temperature (from 20 to 80°C). Ozone charges varied up to 0.2% on pulp. D stages were carried out at 10% pulp consistency in plastic bags placed in a thermoregulated water bath. D Stages were performed at 80°C for 3 hours.

After bleaching, brightness and cel-lulose state of polymerization (DP) were measured according to the ISO standards (ISO 3688 and 5351 respectively). Prior to DP measurement the pulp was reduced with 2% NaBH4 and 1% Na2CO3 at 10% consistency and room temperature for 30 min. Post colour number was the differ-ence (x100) between K/S after aging at 105°C for 24 h and before.

Model compoundsPara-benzoquinone (Lancaster Synthesis), sulphonated naphthoquinone (1,2 naph-thoquinone-4-sulfonic acid, sodium salt, Aldrich), and sulphonated softwood lignin (lignosulfonic acid sodium salt, Aldrich) were commercial products. The molar mass of the “phenylpropane unit” of the sulphonated lignin was 247 g mol-1 as indicated by the supplier. The formulas of the three models are the following:

Reagent,% 0 0.05 0.10 0.20 0.27

ClO2 85.1 - 86.0 87.2 88.0O3 85.1 87.8 88.7 89.5 -

Table I. Final bleaching of a kraft mixed hardwood kraft pulp: bright-ness against chemical charge.

CH2CH

CHCH

OH

O

O

O

OOMe

SO3Na

SO3Na

Parabenzoquinone 1,2 Naphtoquinone-4- sulfonic acid sodium salt

Lignosulfonic acid sodium salt

Reaction of model compoundsAqueous solutions of the model com-pounds were treated at a concentration of

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actually develop some new colouration when they react with small quantities of ClO2. These new chromophores will not be then entirely destroyed with further quantities of reagent. More-over, this study illustrates the fact that quinones, which are also claimed to be possible residual chromophores, are very reluctant to react with ClO2. Therefore, reasons for the low efficiency of chlorine dioxide in last bleaching stages could be either the for-mation of stable chromophores (possibly quinones) when ClO2 reacts on lignin fragments, or the presence of quinones already in unbleached pulp, which will not readily react with ClO2.

Ozone treatment as final bleachingThe use of ozone in pulp bleaching has been implemented in more than 30 bleaching lines worldwide. The justification of introducing ozone in a bleaching sequence is mainly economical. The fact that in theory one mole of ozone (48 g) can exchange 6 electrons against only 5 for 67 g ClO2 makes it a cheaper alterna-tive. Moreover ozone reactivity is such that any unsaturated group in lignin readily reacts with ozone, which is not the case with chlorine dioxide.

Ozone operating conditions have been carefully looked at in order to minimize ozone decomposition, favour lignin degrada-tion, and avoid cellulose depolymerization. It is well documented that ozone delignification should most favourably take place at acidic pH (between 2 and 3) and at the lowest temperature pos-sible. When used in final bleaching the level of pH was not found critical [5] and close to neutral conditions gave good results. This may have something to see with the fact that the target of ozone reaction is then some resistant chomophores, rather than lignin, which would require a different chemistry. In this new application the effect of temperature has never been investigated. However this parameter is of interest since the pulp at this stage must be at least around 70°C.

The following figures refer to the influence of temperature in the ozone final bleaching of the hardwood kraft pulp of initial brightness 83.7% ISO.

Figure 3 shows that, contrary to what was expected, brightness development was better at the higher temperatures, despite the fact that ozone stability should have been less. One reason could be that the oxidized chromophores are more soluble at higher temperature. However attempts to obtain the same effect by sub-mitting the pulp treated by ozone at 20°C to a water extraction at 80°C for 30 min. failed. Therefore it is likely that the higher the temperature the better the chemical degradation of the chro-mophores by the ozone, which remains to be understood.

The better bleaching is obtained at the expense of some cel-lulose depolymerization (Figure 4). However, the loss in DP was moderate and should not modify the mechanical properties of the hardwood pulp significantly.

Finally, the increase in temperature improved the brightness stability of the pulp (lower PC number) (Figure 5). At low tem-perature (20°C) some loss in brightness stability accompanied the bleaching effect of ozone. At 80°C brightness stability was the same as the original one before the ozone treatment. Explanation for this effect has not been found yet.

CONCLUSIONSECF bleaching sequences are based on the extensive use of chlorine dioxide. No other reagent is necessary in the sequence to obtain fully bleached pulps, provided that sufficient charges of chlorine dioxide are applied, even though some oxygen or hydrogen peroxide are added in the extraction stages to reduce the bleaching cost. However a close examination of the ClO2 requirements in the successive D stages shows that ClO2 becomes less and less efficient as the bleaching progresses. In fact, consid-ering the tiny quantities of coloured matters in the pulp before the last bleaching stage, much lower charges of ClO2 should be required.

This study demonstrates that phenolic lignin fragments may

FIG. 1. Behaviour of lignin solution when treated by increasing amounts of ClO2 and O3.

FIG. 2. Colour removal during ClO2 and O3 treatment of quinones in water solution. (a) naphtoquinone (b) para-benzoquinone.

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coloured groups in pulp to bleachability problems, Cellulose Chem. Technol., 40(3-4):243-248 (2006).2. MATEO, C., CHIRAT, C., and LACHENAL, D. The chromophores remaining after bleaching to moderate brightness. J. Wood Chem. Technol., 24(3):279-288 (2004).3. ROSENAU, T., POTTHAST, A., KOSMA, P., SUESS, H.U., and NIMMERFROH, N. Isolation and identification of residual chromophores from aged bleached pulp samples. Holzforschung, 61:656-661(2007).4. LACHENAL, D. Hydrogen peroxide as a delignifying agent, in Pulp Bleaching, Principles and Practice, C.W. Dence and D. Reeve Editors, Tappi Press, Atlanta, :349-359 (1996).5. CHIRAT, C. and LACHENAL, D. Other ways to use ozone in a bleaching sequence. Tappi J. 80(9):209-214 (1997).6. CHIRAT, C., LACHENAL, D., and MATEO, C. Final bleaching with ozonated water, Revue ATIP, 57(2):12-16(2003).7. BROGDON, B.N., MANKCOSKY, D.G., and LUCIA, L.A. New insights into lignin modification during chloride dioxide bleaching sequences (I): chlorine dioxide delignification. J. Wood Chem. Technol. 25:133-147(2005).8. ERIKSSON, T. and GIERER, J. Studies on the ozonation of structural elements in residual kraft lignins. J. Wood Chem. Technol. 5(1):53-84 (1985). 9. BROGDON, B.N. Influence of oxidized lignin structures from chlorine diox-ide delignified pulps on the kappa number test. J. Pulp Paper Science 27(11):364-369(2001).The behaviour of ozone was shown to be very different.

Although new chromophores are also formed when ozone reacts on lignin fragments, those coloured groups will not resist fur-ther ozonation. Also, the quinones models used here were eas-ily degraded by the ozone. Those chemical considerations would explain why the potential of ozone in final bleaching was found better than that of chlorine dioxide.

Ozone delignification is known to work better at lower tem-perature. However, pulp temperature in final bleaching is usu-ally between 70 and 80°C. Contrary to what was expected, ozone bleaching worked much better at 80°C than at 20°C. Several brightness units could be gained only by such an increase in the reaction temperature without substantial consequence on cellulose depolymerization. Moreover, brightness stability was found better after bleaching at 80°C. Therefore ozone final bleaching, which requires neither pH nor pH adjustments, is indeed a very simple and promising process.

ACKNOWLEDGEMENTThis work was carried out in partial fulfilment of the requirements for Guillaume Pipon’s PhD in pulp and paper science at Grenoble INP-Pagora. Wedeco is thanked for their financial and technical contributions to the project.

LITERATURE1. VOIRON, S., KULIGOWSKI, C., and LACHENAL, D. Contribution of various

FIG. 4. Effect of ozone stage temperature on cellulose deg-radation (conditions as on Fig. 3, original DP 1550).

FIG. 3. Effect of ozone stage temperature on pulp bright-ness (pH 3, 0.18% ozone charge, original brightness 83.7%).

FIG. 5. Effect of ozone stage temperature on pulp bright-ness stability (conditions as on Fig. 3, original PC number 0.74).

Reference: LACHENAL, D., PIPON, G., and CHIRAT, C. Final Pulp Bleaching by Ozonation: Chemical Justification and Practical Operating Conditions. Pulp & Paper Canada, April 2009:T48-T51. Paper presented at the 2008 International Pulp Bleaching Conference in Quebec City, Que., Canada, June 2-5, 2008. Not to be reproduced without permission of PAPTAC. Revised manuscript approved for publication by the Review Panel January 5, 2009.

Keywords: PULP BLEACHING, CHLORINE DIOXIDE, OZONE, CHROMOPHORES, TEMPERATURE

Résumé: Pour mieux comprendre pourquoi les derniers points de blancheur sont si difficiles à obtenir lors du blanchiment des pâtes chimiques, alors que les quantités de chromophores résiduels sont extrêmement faibles, plusieurs composés modèles des structures chimiques vraisemblablement présentes dans les chromophores des pâtes mi-blanchies ont été traités par différentes charges de dioxyde de chlore et d’ozone. Selon le modèle (quinones, fragment de lignine) et le réactif utilisé, des résultats différents ont été obtenus. Lorsqu’il réagit avec des fragments de lignine, ClO2 forme de nouveaux chromophores colorés qui vont résister à une attaque ultérieure. L’ozone O3 forme également de nouveaux chromophores mais ceux-ci sont entièrement dégradés par un excès de réactif. De plus, les modèles quinoniques se sont avérés très résistants vis-à-vis de ClO2 alors qu’ils sont facilement dégradés par O3. Ces résultats peuvent expliquer la moindre efficacité du dioxyde de chlore en fin de blanchiment par rapport à l’ozone. Les conditions d’utilisation de l’ozone en fin de blanchiment sont moins critiques que lors d’une délignification. Ainsi il est montré que le stade d’ozonation peut être avantageusement réalisé à haute tem-pérature (80°C).