|[Home] [APPW 2004] [Journal papers]|
PROGRESS IN BLEACHING TO TOP BRIGHTNESS
In ECF bleaching the stability of the pulp's brightness is affected by small residuals of oxidized lignin. Quinoid compounds, generated during chlorine dioxide bleaching are an important source for chromophores producing brightness losses. In hardwood pulp bleaching high temperature in the D0 stage is a relative new tool to improve bleaching results. The two different options for the addition of chlorine dioxide to this hot stage - right in the begin or close to the end - produce similar results only at a high addition level. With lower active chlorine factors the addition in the beginning is more advantageous. In comparison lower Kappa numbers are achieved after a subsequent extraction. Similarly a higher brightness and a better brightness stability results. Obviously the reaction products of chlorine dioxide with lignin do brighten and stabilize pulp brightness during their degradation.
These effects can be transferred to other D stages. Very high temperature and long retention time in a D stage produce improved brightness stability. This temperature increase is most effective in the D2 stage. These effects in the D stages are improved further by a final peroxide stage. This stage produces the best value for brightness stability. High temperature, high charge and high residual of hydrogen peroxide are beneficial. In case the sequence is run with the stages D1-P, high temperature in the D1 stage helps to produce very high final brightness and very low post color numbers.
Conventional conditions in ECF bleaching and high temperature conditions in D stages and P stages in different sequences and positions will be compared with respect to the resulting brightness and brightness stability.
2. Materials and methods
All trials were made with industrial pulp samples taken after oxygen delignification. D and P stages were run in plastic bags in water baths. Ozone was added to well fluffed pulp in a fluidized bed reactor; Eop stages were conducted in a pressurized high-shear mixer. All trials (except ozone) were made at 10% consistency. Brightness was measured with ISO 2470. Reversion testing was made with handsheets prepared at pH 6 on a Buchner funnel with a weight of 280 g/m², Tappi methods UM 200 and T 260 were used.
The dominant bleaching process today is ECF bleaching. It represents more than 90% of world's bleached pulp production. The target of bleaching is a high and stable brightness. Unfortunately, in reality pulp brightness decreases during storage and production of paper. Therefore the intensity of this reversion is an important parameter.
Reversion is the result of chromophores generated by condensation reactions. The active sites required can be the result of poor washing. Indeed, improving washing has a positive effect. However, washing alone doesn't eliminate reversion. Chromophore generation could involve oxidized cellulose. An analysis of different ECF sequences with the "CCOA" method [1 ] showed very little variation of the carbonyl content, however, rather different levels of brightness stability.
An analysis of brightness reversion requires reasonable testing conditions. Tappi's "UM 200" describes aging under moderate conditions, namely a four hours treatment at 105°C in an oven. This removes water rapidly and reversion reactions requiring water will not take place. Therefore this method produces only moderate differences. Another option is the former Tappi test T 260, a test over boiling water for 2 hours. This moist method produces data which correlate better with natural reversion .
Bleaching removes double bonds. This decreases the aging potential. Brightness reversion should be higher for a pulp with an incomplete removal of "lignin". However, things are a bit more complicated. For example, TCF pulps do not have very poor reversion properties despite of a relative high residual of double bonds. Thus the kind of "double bond" seems to be of importance as well. It has been demonstrated earlier , more bleaching chemical produces more brightness and the brighter the pulp the better the brightness stability becomes. There is a very positive effect of more bleaching stages. At constant active chlorine input a four stages sequence has the higher brightness and less reversion. Most pronounced is the positive effect of a final peroxide treatment. Figure 1 has an example.
Fig. 1: Aging of pulp with UM 200 and T 260 after the stages D1 or D2 or P in the sequences D0-Eop-D1-D2 or D0-Eop-D1-P; D0 with Kappa factor 0.23
The post color number  does not compare losses by points of brightness, it uses reflectance and light scattering. The higher the brightness the more negative the same loss in points becomes. Thus differences become even more visible. In Figure 2 the T 260 test data of Figure 1 are shown as post color values. The big improvement in stability with the final P stage is visualized. Only in combination with the final P stage a very low input of chlorine dioxide to the D1 stage is sufficient to achieve a very good final stability. These effects are valid as well for softwood Kraft pulp. After O-D0-Eop-D1-D2 or D0-Eop-D1-P bleaching, reversion of a pine pulp is higher after the D2 stage and lower with a final P stage .
Fig. 2: Comparison of brightness stability after the T 260 test as post color number
The aggressive conditions of hot chlorine dioxide delignification or an application of ozone or of hot peroxide do have an effect on the pulp's viscosity, however, they do not affect the reversion behavior. There is a general positive impact of the use of peroxide as the final bleaching stage .
On the other hand, the scapegoat cited in several papers as being mainly responsible for brightness reversion, hexenuronic acid, does not play an important role in ECF bleaching. Hexenuronic acids are removed in the ECF processes, they are only important in TCF bleaching  or possibly in ECF "light" bleaching . In "real" ECF bleaching other compounds cause reversion. Jääskeläinen  detected significantly more p-quinoid structures in chlorine dioxide bleached pulp compared with peroxide treated pulp. Alkaline peroxide cleaves quinoid structures easily and removes them. In contrast in chlorine dioxide bleaching quinoid structures are generated, their removal is incomplete .
Thus the best way to a stable brightness is a final peroxide bleaching stage. The reaction with chromophores or precursors of chromophores is rather fast. Alkaline conditions in the presence of hydrogen peroxide yield very good results. At 80°C the reaction with chromophores or their precursors needs only 30 minutes to reach the optimum. Lower temperature can be compensated to a certain extent with longer retention time.
5. Further optimization potential
The main responsibility of chlorine dioxide bleaching for brightness reversion leads to the question, whether it would be possible to improve its performance. Conditions generating less quinoid structures would certainly be favorable. An option is the application of more chlorine dioxide. This intensifies oxidation and removal of potential chromophores. It is the simplest way to improve results (see Figure 1), however, it is costly to add more chemical. The stabilization of the pH in a D stage has a positive effect. However, in case higher amounts of chlorine dioxide should be reacted, it is impossible to stop the pH from falling. There are too many acidic compounds generated by the oxidation process.
A recent technical development is the use of very high temperature in the D0 stage. It was first described by Lachenal  and combines chlorine dioxide delignification with hot acid hydrolysis . Initially the intention of its application was to use the hydrolysis reaction which degrades hexenuronic acids to verify chlorine dioxide savings . However, savings are difficult to realize and the more complete delignification achieved with the hot D stage was found to be much more attractive. At high Kappa factor the hot D0 stage allows a significantly lower extraction stage Kappa number compared with the conventional 50°C, one hour approach. This results a "delignification" to just Kappa 3.6, which means twice as many double bonds and bleaching work still to do.
In mill practice two alternatives are available; to start with hot acid hydrolysis and to add the chlorine dioxide only at the very end of the stage. This leaves only several minutes for the chlorine dioxide reaction with the lignin, which is fast at the high temperature. The disadvantage is the similarly fast reaction of ClO2 with the hydrolysis products, furancarbonic acids. Therefore savings can be realized only with a low input of active chlorine. This requires more bleaching action in the D1 stage. The alternative is the addition of chlorine dioxide right at the start of the hydrolysis reaction. Therefore ClO2 will react not only with lignin but also with hexA and the potential for savings in ClO2 with the hydrolysis reaction are similarly decreased. Another disadvantage of this approach is the presence of chloride ions during the high temperature treatment, which might cause corrosion. The comparison of both approaches shows visible differences. Figure 3 has the impact of adding chlorine dioxide in the beginning or at the end of the hydrolysis process. The fast reaction of ClO2 with furancarbonic acid causes a poorer performance for the addition of chlorine dioxide in the end. This disadvantage only vanishes at very high ClO2 input. With high availability of chlorine dioxide Kappa numbers become identical. The start in the presence of ClO2 is an advantage. Chlorine dioxide obviously reacts faster with lignin than with hexA, thus there are enough sites of this compound left for removal by hydrolysis. This is mirrored by the development of the brightness after the subsequent extraction stage (Eop). The reaction with lignin lowers the number of colored sites and increases brightness, which again is most obvious at low active chlorine input ( Figure 4).
Fig. 3: Effect of addition of ClO2 on delignification in a hot D0 treatment; D/A with ClO2 addition in the beginning, 2 h at 95°C; A/D with 110 minutes hydrolysis time at pH<3 followed by ClO2 addition and additional 10 minutes reaction time, all at 10 % consistency
Fig. 4: Impact of addition of ClO2 on brightness, conditions see Fig. 3
The higher brightness is accompanied by a better brightness stability. Figure 5 has these numbers. At low active chlorine input the improvement is very pronounced. Even with a high Kappa factor the advantage of keeping the chlorine dioxide treated pulp at very high temperature is still present. It is a safe assumption, at this temperature level all chlorine dioxide added will be consumed within minutes. Therefore the obvious advantage of keeping the pulp after the reaction with ClO2 for an extended time at above 90°C has to have it's background in additional reactions. It is permitted to speculate with degradation processes involving quinones. Quinones aren't very stable molecules. Provided the temperature is high enough they will react further, either by decomposition or as strong oxidizers towards other compounds present in the pulp. The very visible differences in the AOX load (Figure 6) are in support of the idea of additional oxidation or degradation reactions because of the very high temperature.
Fig. 5: Impact of addition of ClO2 on brightness stability (post color number) after Eop
Fig. 6: Impact of high temperature on AOX load in the stages D0-Eop, bleaching with Kappa factor 0.23
It is consequent to transfer these findings from the D0 stage to the D1 stage. A constant charge of chlorine dioxide is consumed at higher temperature more completely and produces a better D1 stage brightness and reversion. A softwood Kraft pulp, pre-delignified with the stages O-D0-Eop (normal conditions in D0) was used for the test. Figure 7 shows the impact of the temperature increase in D1 from 60°C to 80°C. It is hard to imagine that the moderate increase in ClO2 consumption would be responsible for the improvement. Other reactions, like the oxidation of intermediately formed quinones will have to contribute. The positive impact of these additional reactions is still visible in brightness and reversion improvements after a final P stage. The post color number decreases with the higher temperature in D1 from 1.4 to 0.9. An additional P stage decreases the post color # significantly further to 0.33 and with the high D1 stage temperature to only 0.27 ( Figure 8). Based on the higher temperature in the D stage it is easy to reach more than 90 %ISO, for softwood pulp an important brightness level.
Fig. 7: Impact of temperature on chlorine dioxide consumption, brightness and post color # in a in D1 stage (T 260); softwood Kraft pulp; Eop Kappa 2.2, constant: 1.5% active chlorine, 10% cons., 2h
Fig. 8: Impact of temperature in the D1 stage on brightness and reversion after a final P stage, reversion test T 260; P stage with 0.25 % H2O2, 0.3% NaOH, 80°C, 1.5h, 10% cons.
The effects become even more pronounced with longer retention time. This was tested in a five-stage sequence (D0-Eop-D1-Ep-D2) for the D2 stage (Figure 9) . The small amount of chlorine dioxide is completely consumed after about two hours, therefore brightness changes are very moderate. However, the longer the pulp is kept at 80°C, the less reversion takes place in the aging test. This leads to the question whether an even higher temperature would further improve the results. Bleaching of a softwood Kraft pulp with the sequence D0-Eop-D1-E(p)-D2 and 70°C or 90°C in the D1 and D2 stages resulted in improved brightness and post color numbers (Figures 10 and 11). The benefit is very significant for reversion.
Fig. 9: Impact of extended time in the D2 stage; softwood Kraft pulp, sequence D0-Eop-D1-Ep-D2; constant in D2: 0.5 % active chlorine, 80°C, 10% cons.
Fig. 10: Impact of hot D stages (D1 and D2) in softwood pulp bleaching with D1-E-D2 or D1-Ep-D2; retention time constant at 4 hours in each D stage, all trials at 10% cons.
Fig. 11: Impact of high temperature on post color number (T 260 test)
The advantage of H2O2 addition in the extraction stage between the D stages stays visible not only in a higher brightness but in addition in a decrease of the reversion. Logically, as result of new "quinones" formed in the final D stage the drop is less pronounced compared with a final P stage.
Very high temperature in a D stage obviously degrades compounds present in the pulp, which trigger reversion. This becomes also visible in the comparison of the reversion results using the aggressive T 260 test and the more moderate UM 200 test. Typically a low temperature (around 70°C) D bleached pulp loses more brightness points in humid reversion compared with the dry method. At high temperature obviously potential chromophores are degraded more completely, the differences are evened out (Figure 12).
Fig. 12: Brightness losses in reversion after bleaching softwood Kraft pulp with two or three final stages (-D1-P or -D1-Ep-D2) at normal or very high temperature in the D stages; constant 2% act. Cl in D1, 0.5% in D2, P: 0.5% H2O2, Ep: 0.25% H2O2
The hot conditions do not negatively affect the pulp's viscosity. There is a very small increase of the COD, indicating a minor drop in yield by acid hydrolysis, however, the increase is below 2 kg/t, thus at about 0.1% yield.
It might come as a surprise even to bleaching specialists, however, the positive effect of high temperature in a D stage on brightness stability is not really something new. It is described in literature. It is not mentioned by Rapson  or Reeve  in the "recent" descriptions of chlorine dioxide bleaching technology. However, it is cited by Rydholm . As early as 1955 Harrison and Calkin reported the positive impact of very high temperature in a D stage on brightness stability . Apparently they did not publish - as announced - further work. The obvious difficulty to run a D stage in a mill under the conditions described might have stopped further work. An application of a hot chlorine dioxide stage is not easy. There are practical obstacles. Many chlorine dioxide towers are covered with ceramic tiles to prevent corrosion. The limited stability of the adhesives and the joints typically restricts the temperature of operation to about 80°C. Very long retention time is another problem. In the huge mills built nowadays the dimensions of a tower with several hours retention time would be extreme.
6. Bleaching to very high and stable brightness
Based on these results, it becomes obvious how bleaching has to be modifiied to generate very high and stable brightness. High temperature in bleaching improves the destruction of lignin and other compounds as well as their solubility. High temperature facilitates washing. An effective alternation of the conditions to oxidize at acid pH and extract at alkaline pH requires a four stages bleaching sequence. Starting with a hot D0 stage using a high Kappa factor and an initial addition of chlorine dioxide degrades lignin very effectively. After an oxidative supported extraction stage brightness is already above 82 %ISO and Kappa below 2. The following D1 stage is conducted with a high input of chlorine dioxide and at very high temperature. The positive effect of the high temperature is not directly visible in a better brightness. Figure 13 shows the effect of higher temperature and high active chlorine input on brightness. Brightness increases with the use of more chlorine dioxide. This increase levels off at more than 1.75 % active chlorine. In contrast, higher temperature not necessarily improves brightness, at low ClO2 input brightness decreases with the increase of temperature. This changes with a higher input of chlorine dioxide. The main reason is the improved consumption of the high chemical addition. The impact of trebling the ClO2 amount on brightness is an increase of less than two points. This is the typical moderate effect of a high input of chemical in a short, three stage sequence. This poor picture changes a bit if in addition brightness stability is analyzed. Figure 14 has these data.
Fig. 13: Effect on brightness of a higher input of active chlorine and higher temperature in the D1 stage. Eucalyptus Kraft pulp, D0 with Kappa factor 0.25 at 95°C for 2h, Eop with 0.5 % H2O2, 1.2 % NaOH, 80°C
Fig. 14: Effect of higher input of active chlorine and higher temperature on brightness stability (T 260) as post color #, conditions see Fig. 13
At 70°C even a very high chlorine dioxide input produces a poor brightness stability. Reversion with the humid test procedure stays high. The conduction of the D stage at higher temperature, however, improves post color numbers significantly. The value is cut half at higher input of active chlorine and very high bleaching temperature. Expressed in lost brightness points, loss is as high as 5 points with all levels of ClO2 at 70°C. It decreases to a loss of only 2.8 points (at a higher brightness) for high input and high temperature bleaching. A final peroxide stage further improves the stability of the chlorine dioxide bleached pulp. Figure 15 shows the pronounced improvement of the post color values with a P stage. Stability in a hot humid treatment proceeds to losses below 2 points even with a low input of chlorine dioxide and a low temperature in the D1 stage. The post color values drop from about 0.9 to below 0.3. Even with a low chlorine dioxide charge to the D1 stage but at 90°C the pulp is finally pushed to a post color value at 0.2. This low value can be pushed further down with more chlorine dioxide and a higher treatment temperature in the D stage. Losses in points of brightness decrease to a value below 1 point!
Fig. 15: Post color values (T 260) after an additional final P stage; constant 0.5 % H2O2, 0.4% NaOH, 85°C, 1.5 h
This very high brightness stability goes hand in hand with a very high brightness level. Figure 16 shows the brightness achieved after the final P stage. The data once again confirm the advantage of a four stage sequence over a three stage process. The standard brightness for hardwood pulp of >90 %ISO is achieved easily with a very moderate input of chlorine dioxide in D1 and the final P stage. The potential of this sequence to generate very high brightness with a higher input of ClO2 and a temperature > 80°C becomes visible. A brightness close to 92 %ISO is within reach.
Fig. 16: Final brightness after the P stage, conditions see Figures 14 and 15
These drastic conditions remove the compounds responsible for humid reversion so effectively that the losses (in points) for pulp treated with a higher input of ClO2 at high temperature become smaller than the losses achieved with the dry reversion test UM 200. Figure 17 has an example. Thus quinoid compounds are obviously removed completely. The remaining reversion has a different source, which is yet unknown. These colorforming reactions do take place even in the absence of water.
A five stage sequence can be modified to deliver even higher brightness at very high stability. Instead of using the conventional approach with continuous alternation of acidic and alkaline stages, the D1 and the D2 stage do follow each other. This allows a final P stage and avoids another repetition of the generation of quinoid structures in a final D stage. The full sequence is hotD0-Eop-D1-hotD2-P. The D1 stage is operated at conventional temperature, only D2 is run at very high temperature. The additional hot cleaning step uses an additional small amount of chlorine dioxide (0.5% act. Cl). Figure 18 illustrates the potential to bleach to a brightness around 93 %ISO. The resulting post color number is extremely low.
Fig 17: Comparison of brightness losses in reversion with UM 200 and T 260 of pulp treated with 1.25 % active chlorine in D1 and an additional P stage, data from tests see Fig. 13 - 16
Fig. 18: Brightness development and resulting stability (T 260) with the sequence hotD0-Eop-D1-hotD2-P. Hot D0 with 2.2 % act. Cl, 95°C, 2h, D1 at 75°C, 3h with 1.5 % act. Cl, D2 at 90°C, 3h with 0.5% act. Cl, final P with 0.3 % H2O2, 85°C, 1h
It is necessary to mention that a top brightness requires adequate pulping conditions. Only pulps which have been prepared with sufficient retention time in chip impregnation, a balanced alkalinity, sulfur and temperature range are sufficiently bleachable to reach top brightness. This can be a brightness as high as 94 %ISO.
7. Summary and recommendations
 K. Fischer; Vergilbung von Hochausbeutezellstoff, Papier, 44(10A) V11 (1990)
 A. Potthast, J. Röhrling, T. Rosenau, P. Kosma, H. Sixta; Determination of carbonyl group profiles in cellulosics by fluorescence labeling: Novel application of the "CCOA" method, 12th ISWPC 2003, proceedings I, 147 - 150 Madison, WI
 J.-E. Levlin, L. Söderhjelm, Pulp and Paper Testing, p 128-129, ISBN 952-5216-17-9, Fapet Oy, Helsinki (1999)
 H. U. Suess, C. Leporini; How to improve brightness stability of eucalyptus Kraft pulp, Proceedings, ABTCP Annual conference 2003, São Paulo
 G. Gellerstedt, O. Dahlman, Recent hypothesis for brightness reversion of hardwood pulps; International Colloquium on Eucalyptus Kraft Pulp, Unversidade Federal de Viçosa, Viçosa, MG, Brasil, Sep. 2003, proceedings
 M. Tenkanen, I. Forsskåhl, T. Tamminen, M. Ranua, K. Vuorenvirta, K. Poppius-Levlin; Heat induced brightness reversion of ECF-light bleached pine kraft pulp; 7th European Workshop on Lignocellulosics and Pulp; Proc. 107 - 110 (2002)
 A.-S. Jääskeläinen, A.-M. Saariaho, P. Matousek, A. Parker, M. Towrie, T. Vuorinen; Characterization of residual lignin structures by UV Raman Spectroscopy and the possibilities of Raman spectroscopy in the visible region with Kerr-gated fluorescence rejection; 2003 ISWPC, Madison, WI, proceedings 139 - 142
 J. Gierer; The Chemistry of Delignification; Holzforschung 36(2) 55 - 64 (1982)
 D. Lachenal, C. Chirat; High temperature chlorine dioxide delignification: A breakthrough in ECF bleaching of hardwood Kraft pulps; 1998 Tappi Pulping Conf. Proceedings, 601 - 604, Tappi J. 83 (8) 96, 2000
 A. Maréchal, J. Wood Chem. & Techn. 13 (2), 261 (1993)
 H. U. Suess, C. Leporini; Chemicals demand in ECF bleaching of eucalyptus pulp with extended prebleaching; ABTCP, Evento Branqueamento, São Paulo, May 1998
 W. H. Rapson, G. B. Strumila; Chlorine dioxide bleaching; p 113 f; in: The Bleaching of Pulp, ISBN 0-89852-043-6, Standard Press, Atlanta 1979
 D. W. Reeve, Chlorine dioxide in bleaching stages, p 379- 394 in: Pulp bleaching: principles and practice; ISBN 0-89852-063-0, Tappi Press, Atlanta 1996
 S. Rydholm, Pulping Processes, Interscience Publishers ISBN 0-471-74793-9, p. 983 (1965)
 W. D. Harrison, C. R. Calkins; A study of variables affecting chlorine dioxide bleaching of semibleached sulphate pulp, Tappi J. 38 (11) 641 - 648 (1955)