TAPPI proceedings of the 1988 pulping conference; 1988 October 30-November2; New Orleans, LA. Atlanta, GA: TAPPI Press; 1988: 741-745. Book 3. PROGRESS IN BLEACHING PULPS WITH THE SULFITE-AIR SYSTEM Edward L. Springer JamesD.McSweeny Chemical Engineer Chemist USDA Forest Service. USDA Forest Service. Forest Products Forest Products Laboratory. 1 Laboratory1 Madison, WI 53705-2398 Madison, WI 53705-2398 U.S.A. U.S.A. ABSTRACT The effects of pulp pretreatment. pulp consistency. and quantity of sulfite added on final pulp brightness were studied using the cupric ion catalyzed sodium sulfite-air bleaching system. Aspen kraft pulp that had been partially delignified by chlorination and alkaline extraction was used. Highest brightnesses were attained by pretreating the pulp with cupric ion and by using the highest practical consistency. Brightness increased. but at a decreasing rate, with increases in added sulfite. A search was made for other catalysts. Nickelous ion vas found to be effective. but not quite as good as cupric ion. Spruce and aspen kraft pulps. partially delignified by an oxygen treatment. were easily brightened to semibleached levels using the cupric ion catalyzed sodium sulfite-air system. These results indicate that a chlorine-free bleaching system may be feasible. KEYWORDS Bleaching, Kraft pulp, Sodium sulfite, Copper sulfate.. Nickel sulfate. Populus tremulaides, Picea mariana, Oxygen delignification. 1This article was written and prepared by U.S. Government employees on official time, and it is therefore in the public domain and not subject to copyright. The Forest Products Laboratory is maintained in cooperation with the University of Wisconsin. INTRODUCTION Effluent from the bleach plant is the major source Of pollution from pulpmills that produce bleached kraft pulp (1). Chlorine. hypochlorite. and chlorine dioxide, used in bleaching. react with lignin and other pulp constituents (such as fatty and resin acids) to produce chlorinated organic compounds. Some of these compounds are toxic and mutagenic and are thus harmful to living organisms (1). Significant quantities of these compounds pass through pulpmill waste water treatment systems and are expelled into receiving waters. Because of their harmful effect on the environment. the Swedish and West German govements have restricted the quantities of these materials that can be discharged from their mills. In the near future. the United States will probably also restrict discharges of these materials. A large reduction in the output of chlorinated organic compounds from the bleach plant can be achieved by use of an oxygen delignification Stage after pulping. Complete elimination of these compounds would require the elimination of all chlorine containing oxidants from the bleaching sequence. If chlorine containing compounds were absent from the bleach plant, countercurrent pulp washing all the way back to black liquor recovery could be practiced. thus eliminating all bleach plant effluent. This research is part of an effort to replace chlorine. hypochlorite. and chlorine dioxide in the final stages of bleaching with a suitable nonchlorine oxidizing agent. Our objectives were to optimize the conditions for sulfite-air bleaching of a partially delignified pulp and to demonstrate that oxygendelignified hardwood and softwood pulps can be effectively bleached. SIMULTANEOUS OXIDANT GENERATION AND USE Previously, it was reported that a chlorinated, alkaline-extracted aspen kraft pulp could be bleached by slowly adding a dilute slurry of calcium sulfite to a well-aerated suspension of the pulp (2). However, it was found necessary to first pretreat the pulp with a dilute solution of cupric sulfate. to satisfy its ion exchange capacity, and also to have a trace of cupric ion present in the pulp suspension to act as a catalyst. After extensive testing. this calcium sulfite-air bleaching system was found to be very difficult to control. At times efficient bleaching could be achieved with large increases in brightness: at other times essentially no bleaching occurred under what seemed to be identical reaction conditions. To date. the factor or factors responsible for this variability have not been identified. Because of this problem, all recent work has been done using sodium sulfite. The sodium sulfite-air system has been found to be very easy to control with consistent results obtained repeatedly. This may be due, in part, to the fact that sodium sulfite is added to the reactor as a solution. whereas calcium sulfite (being quite insoluble) is added as a suspended solid. The sodium sulfite-air system was used in studying the effects of pulp consistency (in the reactor). pulp pretreatment. and quantity of sulfite added on the brightness level attained. A chlorinated. alkalineextracted aspen kraft pulp (Kappa No. 6) was used in this work. All Pulp samples were treated after bleaching by soaking them for 4 min in a 0.015% solution of sulfuric acid at a consistency of 1.2%. EFFECT OF PULP CONSISTENCY Three consistency levels, 0.4%, 1.2%, and 2.4%, were studied. Fig. 1 shows the effects of quantity of added sulfite and of pulp consistency on the final bleached brightness. As the amount of added sulfite increased. the brightness increased. but at a decreasing rate. This is similar to the effect of other bleaching agents. For a given level of sulfite added. increasing the consistency increased the brightness level attained. The highest consistency level studied. 2.4%, was at or near the highest level at which efficient mixing could be achieved in the present reactor system. EFFECT OF PULP PRETREATMENT 1988 Pulping Conference / 741
For each consistency level, the pulp pretreatment used. if any. is designated on Fig. 1 by the type of symbol. As in the previous work. some samples of the pulp were pretreated by soaking overnight at 1.0% consistency in a 0.10% solution of cupric sulfate. They were then washed with distilled water before bleaching with the sodium sulfite-air system. The purpose of this pretreatment was to replace the cations on the ion exchange sites of the pulp with cupric ions so that the cupric ions added to the solution in the reactor to catalyze the production of oxidant (peroxymonosulfate) would remain in solution and not be deactivated by being bound to the pulp. To investigate the possibility of avoiding this pretreatment step, samples of unpretreated pulp were placed in the reactor, and an adequate amount of cupric sulfate was added to satisfy both the ion exchange capacity of the pulp and the catalyst requirement. These pulps were then bleached with the sodium sulfite-air system in the same manner as were the cupric ion pretreated pulps. In addition. some samples of the unpretreated pulp were pretreated with acid prior to bleaching. The acid pretreatment procedure was a 15-min soak at 1.2% consistency in a 0.010% sulfuric acid solution. As with the unpretreated pulps. prior to bleaching, sufficient cupric sulfate was added to the solution in the reactor to satisfy both the ion exchange capacity of the acid pretreated pulp and the catalyst requirement. In a previous study on bleaching with potassium peroxymonosulfate. acid pretreatment of pulp was shown to increase the final brightness attained (3). We thought that a similar effect might be observed with the present system. Although the data showed considerable scattering. especially at the 0.4% consistency level, there appeared to be no significant difference in brightness level attained between the acid pretreated pulps and those that had not been pretreated. Pulps pretreated with cupric ion seemed to attain significantly higher brightness levels than the other pulps. This was especially apparent at the 1.2% consistency level. Adding cupric ion to the reactor was apparently not quite as effective as soaking the pulp overnight in a cupric ion solution. Thus. at a given level of sulfite addition, maximum pulp brightness was attained by using the highest possible consistency in the reactor and by pretreating the pulp with a cupric ion solution. EFFECT OF SULFITE CATION The cation associated with the sulfite anion could possibly have a significant effect on bleaching results. The best previously obtained calcium sulfite data for 1.2% Consistency (2) are compared with the present sodium sulfite data an Fig. 1. The calcium sulfite bleached pulps had been pretreated with cupric ion. There seemed to be no significant differences between the sodium sulfite and the calcium sulfite data. These data were, however, taken at different ph levels in the reactor. When employing calcium sulfite. the reactor ph was decreased during the run from 9.0 to 8.5 (2). When using sodium sulfite. the reactor ph was held steady at 12.1. For both systems. reactor temperature was 50 C. An ammonium sulfite trial run gave very poor results. however. the 742 / TAPPl Proceedings reaction conditions had not been optimized. other sulfites have not been tried. EFFECT OF CATALYST To date, Contrary to expectations. some brightness increase could be attained without an oxidant-generating catalyst present in the system. Figure 2 shows the cupric ion catalyzed. unpretreated. and acid pretreated data for 0.4% and 1.2% Consistency together with similar data taken where no catalyst was present in the reactor. At a consistency of 0.4%, substantial brightness increases (up to 17 percentage points) could be obtained without a catalyst present, however, large quantities of sulfite were needed to obtain these increases. These brightness increases may have been due to sulfonation and subsequent solubilization and removal of lignin from the pulp. Watanabe and coworkers (4) found that an acid hydrolysis lignin could be sulfonated and solubilized at low temperatures (70 C) under conditions very similar to those used in our research. It appears that two mechanisms may be acting simultaneously to increase the pulp brightness in the cupric ion catalyzed system: (1) lignin sulfonation and solubilization and (2) lignin oxidation by the generated = peroxymonosulfate anion (SO5 ) with subsequent partial solubilization. On the basis of sulfite consumption per percentage point brightness increase, the sulfite is much more efficiently utilized in the catalyzed system. Although the cupric ion (Cu ++ ) may be found to be the most efficient catalyst. it was suspected that other metal ions (such as Ni ++, Mn ++. Co ++, Zn ++ ) might also be effective as catalysts. To date, we have only studied the use of the nickelous ion as a catalyst. As was done with the cupric ion pretreatment, the pulp was pretreated with nickelous ion by soaking overnight at 1.0% consistency in a 0.10% solution of nickelous sulfate. The pulp was then thoroughly washed with distilled water before bleaching. A dilute solution of nickelous sulfate was added to the reactor so that the concentration of nickelous ion during simultaneous generation and bleaching was 4.0 x 10-5 M. This was the same concentration as was used when cupric ion was the catalyst. The results from the initial trials at 1.2% consistency using nickelous ion are compared with the cupric ion pretreated. cupric ion catalyst results in Fig. 3. For the conditions used. nickelous ion (Ni ++ ) was somewhat less effective than cupric ion. Further work will be done using Ni ++ and other potential catalysts. Metal ion catalysts, such as Cu ++ or Ni ++, have the disadvantage that they are exchanged for the sodium (Na + ) or hydrogen (H + ) ions associated with the carboxyl groups (and other ion exchanging groups) of the pulp and are thus depleted from the reaction solution. The pulps ion exchange capacity must. therefore, be satisfied prior to reaction. This requires a substantial quantity of metal ion. too much to be thrown away or wasted. It would be better to use a catalyst that did not interact with the ion exchanging groups of the pulp. We plan to seek such a catalyst. If the cupric ion (or some other metal ion) proves to be the most effective catalyst. it will be necessary to recover it from the bleached pulp for reuse. In
most laboratory work and on the commercial scale, bleached pulps are acidified with aqueous sulfur dioxide or bisulfite as the last step in the bleaching process. In our work on the sulfite-air system, we have found dilute sulfuric acid to be as effective as bisulfite in this step. Treating a cupric ion catalyzed sulfite-air bleached pulp with acid will displace the cupric ions on the pulp with hydrogen ions and yield a dilute, low-ph solution of cupric ions. If the solution is then drained from the pulp and its ph is adjusted to near neutral with a base (such as sodium carbonate or sodium hydroxide). it can then be used to pretreat a fresh batch of unbleached pulp. Pulp placed in the solution will exchange its sodium or hydrogen ions for the cupric ions in solution. and these ions will be removed from the solution. The solution can then be drained from this batch of pulp and sent on to the recovery cycle. Using this procedure. the cupric ions could be recycled indefinitely. Some losses would undoubtedly occur. and some makeup copper sulfate would be required. however. cupric ion recycle would probably be both technically and economically feasible. The major bleaching costs for the sulfite-air method would thus be the cost of the sulfite and the capital cost of the bleaching reactor and auxiliary equipment. Sodium and calcium sulfites may soon be available at a very low cost, because they are waste materials from certain processes for scrubbing of sulfur dioxide from power plant flue gases for the control of acid rain. BLEACHING OXYGEN-DELIGNIFIED KRAFT PULPS The work described so far has involved the effect of reaction conditions on the bleaching of a chlorinated. alkaline-extracted, aspen kraft pulp. It was. of course. of great interest to investigate the sulfite-air bleaching of oxygen-delignified kraft pulps. Cupric ion pretreated. oxygen-delignified spruce and aspen kraft pulps were bleached using the cupric ion catalyzed, Sodium sulfite-air system. The results from these bleaching experiments together with previous results from bleaching the cupric ion pretreated, chlorinated, alkaline-extracted, aspen kraft pulp are shown in Fig. 4. In all experiments. pulp consistency in the reactor was 1.2%. As can be seen. the sulfite-air bleaching system is effective in bleaching both hardwood and softwood oxygendelignified kraft pulps. This finding opens up the possibility of developing a completely chlorine-free bleaching process for kraft pulps. A two-stage oxygen. sulfite-air bleaching sequence might be both technically and economically feasible for producing semibleached pulps. With further study. it may be possible to attain higher levels of brightness. It appears that much further effort will be required to develop the sulfite-air bleaching method into a reliable industrial process. CONCLUSIONS To obtain maximum brightness increases when using the sodium sulfite-air system, delignified kraft pulps must be pretreated with cupric ion, and a cupric ion catalyst must be present in the reactor. The highest Practical pulp consistency must also be employed. With this system, an oxygen-delignified spruce kraft pulp has been bleached to a brightness of 75% and an oxygen-delignified aspen kraft pulp to a brightness of 82%. EXPERIMENTAL The reactor employed in this work was identical to that used previously (2). It consisted of a 500-ml Pyrex 2 glass beaker containing a fritted glass gas sparging tube and a motor-driven impeller. Temperature, ph, and rate of sulfite addition were carefully controlled. The usual reaction conditions employed are listed below. 2The use of trade or firm names in this publication is for reader information and does not imply endorsement by the U.S. Department of Agriculture of any product or service. Weight of pulp (ovendry basis) 1.3, or 6 g 3 Initial weight of pulp and water 250 g Concentration of Na 2SO 3 solution 5.0 g/1 Rate of addition of Na 2SO 3 solution 30 ml/h Air rate ph 12.1 CuSO 4 added 2,000 ml/min Cu ++ pretreated pulp 1.6 mg H + or unpretreated pulp 16.0 mg Reaction temperature 50 C Delignification of Kraft Pulps Aspen (Populus tremuloides Michx.) kraft pulp, yield 55%, Kappa No. 14, was chlorinated and alkaline extracted as follows: The pulp was reacted with 1.3% chlorine (on dry pulp) at 2% consistency and 25 C for 60 min. Then it was washed and extracted with 2.0% sodium hydroxide at 10% consistency and 60 C for 60 min. Finally. it was thoroughly washed with 70 C water, and then washed with distilled water, and filtered to about 25% consistency. Final Kappa No. was 6. The same sample of aspen kraft pulp was oxygen delignified in a Parr reactor under the following conditions: 3 Depending upon consistency desired. 1988 Pulping Conference/743
Consistency 1.6% NaOH MgSO 4 Total pressure (oxygen+vapor) 2.0% (on dry pulp) 0.5% (on dry pulp) 129 lb/in 2 -gauge Temperature 100 C Time at 100 C 60 min. It was then thoroughly washed and filtered as before. Final Kappa No. was 7. Spruce (Picea mariana) kraft pulp, yield 50%. Kappa No. 30, was oxygen delignified in a Parr reactor under the following conditions: Consistency 1.6% ML88 5514 Figure 1. Effects of pulp pretreatment, consistency. sulfite anion, and quantity of sulfite added on pulp brightness. (ML88 5514) NaOH MgSO 4 Total pressure (oxygen+vapor) 10.0% (on dry pulp) 0.5% (on dry pulp) 120 lb/in 2 -gauge Temperature 125 C Time at 125 C 90 min. It was thoroughly washed and filtered as were the aspen pulps. Final Kappa No. was 9. Determination of Brightness Handsheets were formed on No. 595 Schleicher and Schuell filter paper in a 100-m Buchner funnel. Brightnesses were determined with a Beckman Model B spectrophotometer with a reflectance attachment. Magnesium carbonate, the standard, was set at 100%. LITERATURE CITED ML88 5515 Figure 2. Comparison of catalyzed and uncatalyzed bleaching. (ML88 5515) 1. R. Brannland and G. Fossum. How to cope with TOCl, Proceedings 1987 TAPPI Pulping Conference. p. 243-248, Washington. DC, Nov. 1-5, 1987. 2. E. L. Springer and J. D. McSweeny. Use of calcium Sulfite and air to bleach a delignified aspen kraft pulp. Tappi 69(4): 129 (1986). 3. E. L. Springer and J. D. McSweeny. Bleaching groundwood and kraft pulps with potassium peroxymonosulfate. Proceedings 1986 TAPPI Pulping Conference. p. 671-681, Toronto, ON, Oct. 26-30, 1986. 4. M. Watanabe, M. Sakumato, G. Meshitsuka, and J. Nakano. Radical sulfonation of lignin-water solubilization of acid hydrolysis lignin. Fourth International Symposium on Wood and Pulping Chemistry, Vol. 1, p. 329-333, Paris, April 27-30, 1987. ML88 5516 744 / TAPPI Proceedings Figure 3. Comparison of cupric ion (Cu++) and nickelous ion (Ni++) catalyzed bleaching. (ML88 5516)
ML 88 5517 Figure 4. Comparison of the bleaching of oxygen-delignified spruce and aspen kraft pulps with that of chlorinated and alkaline-extracted aspen kraft pulp. (ML88 5517) 1988 Pulping Conference / 745