Progress in bleaching to top brightness with low reversion

Transcription

1 Progress in bleaching to top brightness with low reversion By H.U. Suess, J. Lally and D. Davies Abstract: High temperature and long retention time in a D stage produces improved brightness stability. Temperature increase is most effective in the D2 stage. These effects in the D stages are improved further by a final peroxide stage, which produces the best value for brightness stability. High temperature, charge and residual of hydrogen peroxide are beneficial. With D1P, high temperature in the D1 stage produces higher final brightness and lower post color numbers. H. U. SUESS Degussa AG Hanau, Germany J. LALLY Degussa Corp. Parsipanny, NJ, USA D. DAVIES Degussa Canada Inc. Surrey, BC dan.davies@degussa.com T H E D O M I N A N T B L E A C H I N G process today is ECF bleaching. It represents up to 90% of the 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 [11] showed very little variation of the carbonyl content, however, different levels of brightness stability. An analysis of brightness reversion requires reasonable testing conditions. Ta p p i s UM 200 describes aging under moderate conditions, namely a four hour 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 that correlate better with natural reversion [3]. 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. Chlorine dioxide in D stages was applied in high purity (>99%). 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 Reversion testing was made with handsheets prepared at ph 6 on a Buchner funnel with a weight of 280 g/m 2, Ta p p i methods UM 200 and T 260 were used. State-of-the-art 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 a relatively high residual of double bonds. Thus, the kind of double bond seems to be of importance as well. It has been demonstrated earlier [7] that more bleaching chemical produces more brightness, and brightness stability improves with the p u l p s brightness. There is a very positive effect of more bleaching stages. At constant active chlorine input, a four stage sequence has higher brightness and less reversion than three stages. Most pronounced is the positive effect of a final peroxide treatment. Figure 1 shows an example. The post color number [7] does not compare losses by points of brightness, instead it uses reflectance and light scattering before and after aging. The same number of lost brightness points gives a higher post color number in the case where the brightness before reversion was higher. Consequently, the differences are made more visible. In Fig. 2, the T 260 test data of Fig. 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 is a very low input of chlorine dioxide to the D 1 stage sufficient to achieve a very good final stability. These effects are valid as well for softwood kraft pulp. After ODEopDD or DEopDP bleaching, reversion of a pine pulp is higher after the D 2 stage and lower with a final P stage. The aggressive conditions of hot chlorine dioxide delignification, an application of ozone or hot peroxide have an effect on the pulp s viscosity, h o w e v e r, they do not affect the reversion behavior. There is a general positive impact of the use of peroxide as in the final bleaching stage [4]. 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 [1] or possibly in ECF light bleaching [5]. In real ECF bleaching, other compounds cause reversion. Jääskeläinen [2] 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

2 BRIGHTNESS FIG. 1. Aging of pulp with UM 200 and T 260 after the stages D1 or D2 or P in the sequences DEopDD or DEopDP; D0 with Kappa factor FIG. 2. Comparison of brightness stability as post number, after the T 260 test. FIG. 3. Effect of addition of CIO2 on delignification in a hot D0 treatment; D/A with CIO2 addition in the beginning, 2 h at 95 C; A/D with 110 minutes hydrolysis time at ph<3 followed by CIO2 addition and additional 10 minutes reaction time, all at 10% consistency. FIG. 4. Impact of addition of CIO2 on brightness; for conditions see Fig. 3. quinoid structures are generated, and their removal is incomplete [12]. 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. Further optimization potential The strong impact of chlorine dioxide bleaching on brightness reversion leads to the question whether it would be possible to improve its performance. Conditions generating fewer 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, as shown in Fig. 1, however, it is costly to add more chemical. The stabilization of the ph in a D stage has a positive effect. However, in the case where larger 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 Pulp & Paper Canada T 205 use of very high temperature in the D0 stage. It was first described by Lachenal [6] and combines chlorine dioxide delignification with hot acid hydrolysis [10]. Initially the intention of this application was to use the hydrolysis reaction, which degrades hexenuronic acids, to achieve chlorine dioxide savings [8]. 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 (95 C) D0 stage leads to a significantly lower extraction stage kappa number. The kappa number after a prolonged 2-hour treatment at 95 C in D0 (factor 0.25) is at 1.6, Fig. 3. A conventional D0 treatment at 50 C, one hour, leads with identical active chlorine input to an Eop kappa of 3.6, which means twice as many double bonds and requires much more additional bleaching action to reach top brightness. In mill practice two alternatives are available. One is 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 illustrates 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, Fig :10 (2005) 41

3 BRIGHTNESS FIG. 5. Impact of addition of CIO2 on brightness stability (post color number) after Eop. FIG. 6. Impact of high temperature on AOX load in the stages D0Eop, bleaching with Kappa factor 0.23, 1% active chlorine in D1. FIG. 7. Impact of temperature on chlorine dioxide consumption, brightness and post color # in a in D1 stage (T 260); softwood Kraft upulp; 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. FIG. 9. Impact of extended time in the D2 stage; softwood Kraft pulp, secquence D0EopD1EpD2; 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 D1ED2 or D1EpD2; retention time constant at 4 hours in each D stage, all trials at 10% cons. The higher brightness is accompanied by a better brightness stability, as seen in Fig. 5. 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 that at this temperature level all chlorine dioxide added will be consumed within minutes. Therefore, the obvious advantage of keeping the pulp at above 90 C for an extended time after the reaction with ClO2 has to have its background in additional reactions. It is permitted to speculate on degradation processes involving quinones, as they are :10 (2005) not 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, Fig. 6, support the idea of additional oxidation or degradation reactions because of the very high temperature. Consequently, these findings may be transferred from the D0 stage to a 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 ODEop (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 must 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 number significantly further to 0.33, T 206 Pulp & Paper Canada

4 FIG. 11. Impact of high temperature on post color number (T 260 test). FIG. 13. Impact of high temperature on the AOX load generated in softwood Kraft pulp bleaching. Constant 3.35% active chlorine, 1h and ph <3 in D 0, variable temperature (50 C or 90 C); Eop with 1.8% NaOH, 0.5% H 2 O 2 and 0.4 MPa O 2 ; D 1 with 1% active chlorine, 2 h, and 70 C or 90 C. and with the high D 1 stage temperature, to only 0.27, Fig. 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. The effects become even more pronounced with longer retention time. This was tested in a five stage sequence (DEopDEpD) for the D 2 stage, Fig. 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 DEopDEpD, with 70 C or 90 C in the D 1 and D 2 stages, resulted in improved brightness and post color numbers, Figs. 10 and 11. The benefit is very significant for reversion. The advantage of H 2 O 2 addition in the extraction stage between the D stages stays visible not only in a higher brightness but also in a decrease of the reversion. Logically, as a result of new quinones formed in the final D stage the drop is less pronounced compared with a final P stage. Ve ry high temperature in a D stage obviously degrades compounds that trigger reversion in pulp. This also becomes 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 FIG. 12. Brightness losses in reversion after bleaching softwood Kraft pulp with two or three final stages ( D 1 P or D 1 EpD 2 ) at normal or very high temperature in the D stages; constant 2% act. CI in D 1, 0.5% in D 2, P: 0.5% H 2 O 2, Ep: 0.25% H 2 O 2. high temperature obviously potential chromophores are degraded more completely, so the differences are evened out, Fig. 12. The hot conditions do not negatively affect the pulp s viscosit y. There is a very small increase of the COD, indicating a minor drop in yield because of acid hydrolysis, however, this increase is below 2 kg/t. This translates into about 0.1% yield. It might come as a surprise, even to specialists in bleaching, that 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 [13] or Reeve [9] in the recent descriptions of chlorine dioxide bleaching technology, however, it is cited by Rydholm [14]. As early as 1955, Harrison and Calkin reported the positive impact of very high temperature in a D stage on brightness stability [15]. 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 e a s y. There are practical obstacles. Many chlorine dioxide towers are covered with ceramic tiles to prevent corrosion. The limited stability of the adhesives and of the joints typically restricts the temperature of operation to about 80 C. Ve ry 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. However, while it might be difficult to apply very long retention time in a D 1 or a D 2 stage, an operation of the D 0 stage at higher temperature is more easily achieved. Figure 6 demonstrates a very interesting side effect of the high temperature D 0 treatment of hardwood kraft pulp. The AOX generated was cut in half. It is rather attractive to test whether this would be valid for the delignification of softwood pulp as well. An oxygen delignified softwood kraft pulp was treated with an identical amount of active chlorine (kappa factor 0.23) at 50 C or alternatively at 90 C. The retention time was held constant at 1 hour. The impact on the AOX residual in the effluent is significant. While the conventional treatment produces kg/t of AOX, the high temperature D 0 stage has less than half of this amount - only kg/t AOX were measured. Obviously a significant part of the halogenated compounds is thermally unstable and degrades because of the high temperature. Figure 13 shows this comparison. Because softwood pulp does not contain hexenuronic acids, there is no impact on the kappa number after Eop - it remains at the same level. Within the bleaching sequence, brightness stability is affected by the hot D 1 or D 2 stage, thus the lower AOX load is the visible advantage of high temperature D 0. If combined with high temperature in the D 1 stage, another benefit is a lower residual of halogenated compounds in the pulp. The OX decreases from 170 g/t with conventional temperatures in a DEopDD sequence to just 120 g/t or with DEopDP from 130 g/t to 110 g/t. Thus even normal

5 FIG. 14. Effect of higher input of active chlorine and higher temperature in the D 1 stage. Eucalyptus Kraft pulp, D 0 with Kappa factor 0.25 at 95 C for 2 h, Eop with 0.5% H 2 O 2, 1.2% NaOH, 80 C. FIG. 15. Effect of higher input of active chlorine and higher temperature on brightness stability (T 260) as post color #, conditions see Fig. 13. FIG. 16. Post color values (T 260) after an additional final P stage; constant 0.5% H 2 O 2, 0.4% NaOH, 85 C, 1.5h. FIG. 17. Final brightness after the P stage; for conditions see Figures 14 and 15. amounts of chlorine dioxide can produce ECF-light effects. Bleaching to very high and stable brightness Based on these results, it becomes obvious how bleaching has to be modified 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 stage bleaching sequence. Starting with a hot D 0 stage using a high kappa factor and an initial addition of chlorine dioxide degrades lignin very e f f e c t i v e l y. After an oxidant-supported extraction stage brightness is already above 82 %ISO and kappa below 2. The following D 1 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 14 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 does not necessarily improve brightness - at low ClO 2 input, brightness decreases with the increase in 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 ClO 2 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, Fig. 15. At 70 C, even a very high chlorine dioxide input produces poor brightness stability. Reversion with the humid test procedure stays high. Conducting the D stage at higher temperature, however, improves post color numbers significantly. The value is cut in 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 ClO 2 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 16 shows the pronounced improvement of the post color values with a P stage. Stability in a hot humid treatment gives losses below 2 points even with a low input of chlorine dioxide and a low temperature in the D 1 stage. The post color values drop from about 0.9 to below 0.3. Even with a low chlorine dioxide charge to the D 1 stage but at 90 C the pulp is finally pushed to a post color value of 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! This very high brightness stability goes hand in hand with a very high brightness level. Figure 17 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 with a final P stage. The potential of this sequence to generate very high brightness with a higher input of ClO 2 and a temperature > 80 C becomes apparent. A brightness close to 92 %ISO is within reach. 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 ClO 2 at high temperature become smaller than the losses achieved with the dry reversion test UM 200. In the example shown in Fig. 18, quinoid compounds are obviously removed completely. The remaining reversion has a different source, which is yet unknown. These color forming reactions take place even in the absence of water. A five stage sequence can be modified

6 FIG. 18. Comparison of brightness losses in reversion with UM 200 and T 260 of pulp treated with 1.25% active chlorine in D 1 and an addtional P stage, data from tests see Fig FIG. 19. Brightness development and resulting stability (T 260) with the sequence D hot EopDD hot P. Hot D 0 with 2.2% act. CI, 95 C, 2h D 1 at 75 C, 3h with 1.5% act. CI, D 2 at 90 C, 3h with 0.5% act. CI, final P with 0.3% H 2 O 2, 85 C,1h. to deliver even higher brightness at very high stability. Instead of using the conventional approach with continuous alternation of acidic and alkaline stages, the D 1 and the D 2 stage 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 D hot EopDD hot P. The D 1 stage is operated at conventional temperature, only D 2 is run at very high temperature. The added hot cleaning step uses an additional small amount of chlorine dioxide (0.5% act. Cl). Figure 19 illustrates the potential to bleach to a brightness around 93 %ISO. The resulting post color number is extremely low. It is necessary to mention that top brightness requires adequate pulping conditions. Only pulps that 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. Summary and recommendations Quinoid structures appear to be the compounds most important for reversion. The elimination of these remaining quinoid structures is easily achieved with a final P stage. Chlorine dioxide bleaching gives lower brightness stability. However, very high temperature (90 C) in any D stage will improve results. Brightness stability is better and AOX load as well as OX residual decrease. Very high brightness requires effective removal of all impurities. A five stage sequence (D h o t E o p D D h o t P) with high temperature (90 C) in the D2 stage and a final P stage results in a final brightness >92 %ISO and very low losses in reversion. L I T E R AT U R E 1. GELLERSTEDT, G., DAHLMAN, O. Recent hypothesis for brightness reversion of hardwood pulps. Proceedings International Colloquium on Eucalyptus Kraft Pulp, Unversidade Federal de Viçosa, Viçosa, MG, Brasil, Sep. (2003). 2. JÄÄSKELÄINEN, A.-S., SAARIAHO, A.-M., MATOUSEK, P., PARKER, A., TOWRIE, M., VUORI- NEN, T. Characterization of residual lignin structures by UV Raman Spectroscopy and the possibilities of Raman spectroscopy in the visible region with Kerrgated fluorescence rejection. Proceedings 2003 ISW- PC, Madison, WI, (2003). 3. POTTHAST, A., RÖHRLING, J., ROSENAU, T., KOSMA, P., SIXTA, H. Determination of carbonyl group profiles in cellulosics by fluorescence labeling: Novel application of the CCOA method. Proceedings 12th ISWPC 2003, I, Madison, WI. (2003). 4. SUESS, H.U., LEPORINI, C. How to improve brightness stability of eucalyptus Kraft pulp, Proceedings ABTCP Annual conference, São Paulo (2003). 5. TENKANEN, M., FORSSKÅHL, I., TAMMINEN, T., RANUA, M., VUORENVIRTA, K., POPPIUS-LEVLIN, K. Heat induced brightness reversion of ECF-light bleached pine kraft pulp. Proceedings 7th European Workshop on Lignocellulosics and Pulp; (2002). 6. LACHENAL, D., CHIRAT, C. High temperature chlorine dioxide delignification: A breakthrough in ECF bleaching of hardwood Kraft pulps; 1998 Tappi Pulping Conf. Proceedings, , Tappi J. 83 (8):96 (2000). 7. LEVLIN, J.-E., SÖDERHJELM, L. Pulp and Paper Testing, p , ISBN , Fapet Oy, Helsinki (1999). 8. SUESS, H.U., LEPORINI, C. Chemicals demand in ECF bleaching of eucalyptus pulp with extended prebleaching; ABTCP, Evento Branqueamento, São Paulo, May REEVE, D.W. Chlorine dioxide in bleaching stages, p in: Pulp bleaching: principles and practice; ISBN , Tappi Press, Atlanta (1996). 10. MARÉCHAL, A. J. Wood Chem. & Techn. 13 (2):261 (1993) 11. FISCHER, K. Vergilbung von Hochausbeutezellstoff, Papier 44(10A):V11 (1990) 12. GIERER, J. The Chemistry of Delignification. Holzforschung 36(2):55-64 (1982). 13. RAPSON, W.H., STRUMILA, G.B. Chlorine dioxide bleaching; p 113 f; in: The Bleaching of Pulp, ISBN , Standard Press, Atlanta (1979). 14. RYDHOLM, S. Pulping Processes, Interscience Publishers ISBN , p. 983 (1965). 15. HARRISON, W.D., CALKINS, C.R. A study of variables affecting chlorine dioxide bleaching of semibleached sulphate pulp. Tappi J. 38 (11): (1955). R é s u m é : L utilisation d une température élevée et d un temps de rétention long à l étape D a permis d améliorer la stabilité de la blancheur. La température plus élevée est le plus efficace à l étape D2. Ces effets aux étapes D sont encore davantage améliorés par une étape finale au peroxyde, qui offre la meilleure valeur en matière de stabilité de la blancheur. La température élevée, les charges et le peroxyde d hydrogène résiduel offrent des grands avantages. Avec D1P, la température élevée à l étape D1 produit une meilleure blancheur finale et réduit l indice de jaunissement. Reference: SUESS, H.U., LALLY, J., DAVIES, D. Progress in bleaching to top brightness with low reversion. Pulp & Paper Canada 106(10): T (October 2005). Paper presented at the 91st Annual Meeting in Montreal, QC, Canada, February 7-10, Not to be reproduced without permission of PAPTAC. Manuscript received December 14, Revised manuscript approved for publication by the Review Panel March 29, Keywords: BRIGHTNESS, BLEACHED PULPS, LIGNINS, HARDWOODS, HIGH TEMPER- ATURE, MULTISTAGE PROCESS, BLEACHING, CHLORINE DIOXIDE.