ChilOtCr TV

of the chlorine was replaced with equivalent chlorine dioxide (7). However.m 1963 it WAS reported that substantial substitution of chlorine by chlorine dioxide resulted in a synergistic effect so that delignification Dl g i l ai n ei nfc to efficiency was greatly improved (6). Fur ther improvement was achieved when the 1 Introduction ehlorine dioxide and chlorine were added „, scqueni.iiiUv,chlorincdioxide first,with 50% -Huchlorine. a mixture of chlorine dioxSubsmulionhavinglhe g^^ efficiency ide and chloric generated from the reduc(8)^ rechnk,cdidnotgainwideac. lion of potassium chlorate by hydrochloric Ccpranceunti, tne,a(cm0swhcnenvi. acid, was first described by Davy in 1811 ronmcnta| concerns about chlorinated who noted that it destroyed the color of ^^ andotherchIormaiedorganicma. vegetable dyes (I). Watt and Burgess patIerja|ы,h ы^ R)dccreascchJorme e emed the bleaching oi soda wood pulp with ^ an(|Д(]tsu,>stanliai, am, ,atercom. chlorine cuchlorinc and sodium hydrox- ^ laccmcmwitnch|orincdioxide. ide m lH5i (2). In .he 1920s. Schmidt and „uhb'chapter. .he subjects discussed are: co-workers demonstrated that chlorine diрюсещchemisIrVtflowsheets, and varioxide does not react with carbohydrates ab|cs;|pquaHtiesSBchas brightness, Q>. m the 1930s, the Mathicson Chemical streng|h)and cleanliness; dioxins in pulp Corp. introduced a process tor bleaching andcffluenl; effluent qualities such as orin which sodium chlorite was added to a ganochJorine content, toxicity, and color; pulp suspension and gaseous chlorine was andfin^ economics. There have been sevitdded to "activate- the chlorite, that is, to cra, revj(.wsonch|orincdi4)xJdcde|ignifigenerate chlorine dioxide. The first Induscationmwhjchthese ^ aredi^llssed trial processes for continuous generation (9-14) and use of chlorine dioxide for pulp bleach ing began in 1916 with almost simuita2. Delignification chemistry and neous,though independent,developments kinetics in Canada and Sweden (4, 5>. In one early _ 1n,. ZmlDefin,tl0ns application, chlorine dioxide was used as the sole bleaching agent, rcplacmg chlorine Chlorine dioxide is an oxidant which for delignification and, in later stages for accepts five electrons per molecule in bcbleaching, to facilitate resin removal and for >nS reduced lo chloride ion: the preparation of pure alpha cellulose from CIO +■ 4 H++ 5 с —* CI" + 2 H ОИ] .... , . ., ,. , , . ir LeJL.iA^,. .^., »i .лоп, 1 he molecular weight ol chlorine dioxide However, from 1946 on into the 1980s. , ,_ _ °, , ,,.,,_ .. , , is 67.-> and so its equivalent weigh, is 13-:> chlorine dioxide was not used extensively .,_ _ _, -, , , _ .. , ,_____ „ .,..*.,, •, .,' (6/.5 + 5)- Chlorine accepts two electrons for del.gn.fica ,on bu was main уused for when ^^ l(y^^ hasдш> bleaching m the final stages- Chlorine di|ecu|ar m()f?, ^ ^ ^ an-Vi|. oxide was more expensive than chlorine or ]em |uof„ 5(?, +2). calcium hypochlorite, but it enabled pro duction of high brightness kraft pulp withCl2+ 2 e' —* 2 СГ (2] out loss of pulp strength. Indeed, it was Therefore, in substituting chlorine dioxide these- advances in pulp bleaching technology and chlorine dioxide generation which stimulatcd the growth of the bleached kraft industry. Industrial use of chlorine dioxide to replace chlorine was at first only a means of protecting the strength of the pulp; 5-10% fi>r chlorine to provide equivalent electron transfer, one weight unit of chlorine dioxide can replace for 2.63 weight units (35.5 * 13.5) of chlorine. "Equivalent chlorine," also known as acdvc chlorine, is a common unit of oxidant in bleaching technology; one weight unit sulfite pulp (6). C ilOC T h t r V --* , t-lH flllC O I) jo i( ie If! x ** ? of chlorine dioxide is equal to 2.63 weight units of "equivalent chlorine.'' Chlorine dioxide substitution,expressed as a percent, is also a very common unit and is based on equivalent chlorine. Equivalent chlorine applied on pulp is sometimes expressed as "kappa factor." also known as (equivalent or active) chlorine multiple: [chlorine + chlorine dioxide applied! = as % equivalent chlorine on pulp [kappa1 unbleached pulp] (31 X /actor kappanum ber_ ] As an exam ples kappa pulp treated w ith 25 chlorineand chlorine dioxide corresponding to a kappa factor 0.20 at 50% substiof tution would be treated with: 25 x 0.20 = 5.0% equivalent chlorine 0,1pulp 2.2 Reactions occurring during chlorine dioxide delignification Chlorine dioxide is reduced through a series of steps involving several intcrmediates before chloride ion is produced. Нуpochlorous acid (HOC!) and chlorine <Clp are among these intermediates, and their presence can lead to the formation of chlorinatcd organic matter. Another undesirable by-product is chlorate ion (CIC\") which is no< ««active and so its form ation m eans loss of bleaching efficiency. Fi urc S ' Illustrates the changes in oxidalionstaleand taction pathways of the reactive intermediates during chlorine dioxide bleaching of pulp (15). Chlorine dioxide reacts with pulp, transferring,in the » process, one electron to produce chlorite ion (CIO /) by reaction (1| (Fig. hlo1). C ^оечnolreacldirccl|vwilnpuip riteion Chlorine dioxide also reacts with (2} pulp is. to form H OCI, w hich in part, converted to(:1[>vhydrolvsis {*,). Hvpochlorous acid ^нchlorine react with'pulp producing chloride ion and chlorinated organic matter (4 to 7). Chlorine reacts with chlorite Юregenerate chlorine dioxide (10), while hypochlorous acid reacts with chlorite to = 50kg equivalent chlorine per m etricton ol pulp = 100lb equivalent chlorine per short ton of pulp which would he made up of 5.0x0.50 = 2.5% chlorine on pulp and 5.0x0.50/2.65 ■ 0.95% chlorine dioxide on pulp. form chlorate ion (JQOp (8} (16). Under acidic conditions- chlorite decomposes to chlorine dioxide and chloride ion (9) (16). The pathways described above and the mechanisms of reaction of chlorine dioxide with lignin and with other pulp constituenls are discussed in depth in Chap. Ш 3 (125). Much of the literature on chlorine dioxide delignification concerns manipulation of conditions to influence the reactions illustraied in Fig. 1; for example, to decrease chlorate formation and thereby increase cfflciency or to decrease chlorine formation so as to decrease organochlorine formation. tion of oxidant and the rate of delig- nification are extremely high. Fven at the relatively low temperature of 10°C, 90% of the chlorine and 60% of the chlorine dioxidc are consumed in only ten minutes (Fig. 2),whileat50°Cvirtiiallyallofeitherchemical is consumed in ten minutes (Fig. 3). At 104;. the rate is low enough so that chloгшеcanbe observed to react faster than chlorine dioxide; both the rate and extent <>f delimitation with chlorine dioxide arc significantly less. At 50°C, the rates of clilorinetUoxide consumption and delignificationaR. indistinguishable from those of chIorinc;,, w/50mixtnrcofcnlorincШ chlorine dioxide <!>+<-) reacts almost a» rapidl as 2.3 Delignification kinetics > chlorine. —, ,, f. , Extensive kinetic studies bv Gcrmgard r The earliest studies of the rate of . , , ,- ,* .. •,» . .. ... ^ ,, , ,. .. . show that the rate of e delignification with delignification using chlorine dioxide, alone ,, . .. ., , -,, , dl nnc<,iOX, ,s and in combination with chlorine, were ° f JJ* °rderW"h"Ч** ,0 reported by Hatton in 1966 (17). The re^ mimber(lfi}: -0 suits for treatment of softwood krali pulp ц- _ ^ \c\o ]"' III'l - FCIT'* ЮГ41 having a 19.5 permanganate number (kappa mimber of approximately 30) are shown in Figs. 2 and 3. The same amount of cquivalent chlorinc was applied for each case (арproximately 0.18 kappa factor). As is typical of such Studies, after the chlorine dioxide/ chlorine treatment, the pulp is treated with ........' . ., alkali шan extraction stage and the extent of delignification is measured after the extraction stage. The initial rate of consumptl , wncre , к= rate constant (influenced by the type of pulp, the bleaching temperature, and the extraction stage conditions) ,, ,,.____,. t = bleaching tune К = pulp kappa number determined after an alkaline extraction stage. This is an empirical, rather than mechanistic, kinetic expression which represents a large number of simultaneous and sequential reactions between chlorine dioxide and lignin. The activation energy is in the range 52 to 66 kl/mole, /.*?., the rate doubles for each rise in temperature of 10"C. From the equation it is apparent that the rate can also be increased by increasing the chlorine dioxide concentration, by increasing the chloride ion concentration, and by decreasing the hydrogen ion concentration. This equation applies to conventional and "modified" unbleached kraft pulps. Oxygcn-d dignified pulps react more slowly and to a lesser extent as shown in Fig. 4 (19). When both chlorine and chlorine dioxide arc present, the rate of delignification can be expressed by (20): -dJC.= klCIOJn|ClJmlO di г 15] where k= rate constant (proportional to K\ where Kt is the initial kappa number), dependent on the type of pulp and temperature) И = 0.5 for unbleached kraft and 0.0 for oxygciMlcIignificd pulps m= 0.5 for IC121 < 1.0 mmole/L, and 0.0 for [Су> 1.0 mmole/L. it was not until 1963 that it was shown that when chlorine dioxide was substituted for chlorine. 3. delignification efficiency increased significantly (6). chlorine disappears proportionately taster than does chlorine dioxide (21). 22.The activation energy is 56 kj/molc Where mixtures of chlorine and chlorine dioxide are used. This was confirmed in other . 23). Delignification efficiency 3.1 Degree of substitution and mode of addition Although the use of mixtures of chlorine dioxide and chlorine to bleach pulp had been described in the literature on numerous occasions (2. As mentioned earlier. 8 for oxygen-delignified eucalyptus kraft pulp (33)-1" the case oroxygen-delignified softwood pulp. a kappa factor0. it was discovered that when the chlorine dioxide and chlorine were added to the pulp scqucnlially. In going from softwood to hardwood and from unbleached kraft to oxygen- delignified pulp.17 (50 kg) is required at 40%chlorine dioxide. When chlorine was added first. to reach an extracted kappa number of 6. Chlorine dioxide delignification of kraft unbleached and oxygen-delignified pulps are compared in Fig. the chlorine dioxide first.20(60 kg of equivalent chlorine per metric ton of pulp) is required at 10% chlorine dioxide substitution but only 0. For example. and most dramatically. illustrating the relative difficulty in dclignifying these pulps with chlorine dioxide. Further. 5 for softwood kraft pulp (30). 25. 27-29). for oxygcn-dclignificd eucalyptus pulp. Figure Уshows the benefit of 404)0% chlorine dioxide substitution in the (DC) sequential mode 04). the free phenolic content of pulp decreases thereby Increasing the difficulty of del ign ill cation with chlorine dioxide alone.reports (77. the final kappa number is higher for 100% chlorine dioxide compared to 100% chlorine. 24. Maximum delignification efficiency was achieved when the fraction of chlorine dioxide was between 40 and 60% (8. in Fig. the benefit was even greater (#J. delignificaiion efficiency was worse. 10 where the chlorine dioxide consumed per metric ton of pulp is plotted against kappa number decrease achieved (35). oxygenbleached pulps are not as responsive to . birch pulp. 7 for birch kraft pulp 02). This is illustrated in Fig. Лuseful approach for quantifying the efficiency of delignification has been used extensively by Germgard. 6 for oxygendelignified softwood kraft pulp (3 !).0. 26). and in Fig. in Fig. kappa number.. WW.. pulp and chemical concentration. [CX]. It was found that. groups in the pulp.the type of pulp. . and to neutralize phenolic and other acidic . and for oxygen-deligmhedfokraft pulp M to temperature. the chemical rcquircm ni * increased 06).but at pH greater than 3 the chlorine dioxide requirement increased.at pH greater than 2. pH had no influence between 1. Inaddition.d water solubility of ligninfwgmcntsMdcscribedinnloredetail in the chapter on extraction (125X When l chlorine dioxide replaces chlorine. 26. 37.P-wllicncreates new phenolic groups. Estimates of the sodium hydroxide required have been reported by several authors for softwood pulps (25..chlorine dioxide as are unbleached kraft pulps.4 to 10%. and the extraction stage conditions) .. chloride ion concentration.ire uiredfor д<:io = к[H*]-*w (K-05.«. . chlorine dioxide conccntration from 1 to 5 mmole/L and pulp consistency from 0. the chlorine dioxide requirement doubles. К = kappa number ol the starting pulp ° ■ K = kappa number of the treated pulp aftcr alkaline extraction..К■**) (7] * 4 extraction Sodium hydroxide is required in the exwhere traction stage to solubilizc lignin made susДCIO = CIO consumption to reach target ccptible to alkali in the previous stage. temperature from 25 to 604:. if the chloride ion conccntration is lower than 58 mg/L. = к[H*]"° in (K/K) [6] . which leads to lonization ^ mcrcas<. ' .2 and 3. а 50/50 mixture.. pH had no influence between 1. К к = a constant (influenced by pH. . 28. The equations below permit euleulation of the chlorine dioxide required: for unbleached kraft pulp ДCIO. Several variables had no effect on deli gn ill cat ion efficiency. less hydrochloric acid is produced in the first stage and there are fewer phenolic groups and less organically bound chlorine in the pulp. . 3 2Alka.dependingonthe wayin which the chlorine dioxide is used.less equivalent chlorine may be needed in the first stage and less sodium hydroxide again is required.5 and 2 but. A study of the delign 111 cation efficiency of chlorine dioxide/chlorine mixtures showed very similar behavior with respect Sodium hydroxide acts in a number of ways: ° """"J* "£T^ K°"' T ^T sta c:1о *? &<**>№* chiorme bound to the PU. and the responses of oxygcn-delignified and un^ea^ea pulp. after alkaline extrac- tion.because chlorine dioxide oxidizes (and bleaches) lignin to a greater degree than does chlorine. in the delignification of softwood kraft pulp having a 30 kappa number. Hquimolar concentrations of chlorine dioxide and chlorine were used for the (D+QB case.thc brightness increase is greater with increasing degree of chlorine dioxide substitution (39). for three softwood kraft pulps (28).38) and hardwood pulps (26). 13. that it is an accurate measure of the amount of bleaching agent required in later stages. However. It has been argued that. Figure 12 indicates the sodium hydroxide consumed per equivalent chlorine consumed in the first stage. Figure 11 shows sodium hydroxide consumed in the extraction stage. gives a false measure of lignin content. kappa number. as a runetion of chlorine dioxide substitution.increases wilh degree of delignification with the exception of the initial phases of chlorine-only delignification. 3. brightness. As shown in Fig.2% on pulp (24 to 18 to 12 kg per metric ton of pulp).4 to 1. for both softwoods and hardwoods (26).Йto 1. for example. The discussion of delignification in the previous section is predicated on the assumption that the kappa number is a true measure of progress toward fully bleached pulp and.increasing the percent chlorine dioxide in the first stage from 10 to 50 to 100 would decrease the sodium hydroxide required (not including acid carryover in the filtrate) from 2. As an example. in particular.3 Brightness development Brightness development actually begins during delignification with combinations of chlorine and chlorine dioxide. an oxidative permanganate test. it has been clearly shown that the post-extraction kappa number is linearly correlated . DC.24 kappa factor. are used in the first stage (44). and the m. The results arc given in Table 1 for several pulps (40-43.27 kappa factor.evemem of high brightness is complicated by the variety of chemicals and sequences used and the range of conditions*pplied that can be used for bleaching partially dehgnified pulp and by the diversity of criteria available by which to judge a bleaching sequence. Another optimization siudv.SO brightness. . Increasing substitution to 70-90% required a significant increase in chemical and led to a significant increase in cost.tervalbciween Dand Сwas mcreased from 2ю4п^пц^^ m^ ^ ^^ ^ es ^ comlitionshadnoщ^псеon costs at 30^0%chlorincdioxide. Germgard and co-workers have optimized (DOEDED sequences by adjusting the kappa factor in the first stage to give Uie lowest overall equivalent chlorine use.____.„. Optimizaiion for lowest chemical cost was also studied and was usually close to the optimal conditions for lowest overall equivalent chlorine. „ . For unbleached softwood pulp. costs increased at 0. .de use in deligmficallon on the ach... The results of a similiar «udy on optimization of (DC)<E+0)D bleaching of oxygen4lclignmcd eucalyptus arc summarized in Table 2 (33).20-0. Figure 14 shows the brightncss advantage gained.20 to 0.27 (46). The synergistic effect of chlorine dioxide in combination with chlorine in the first stage is apparent from the brightness reached in later stages. is used in the first stage of аДОEH sequence . the lowest chemical cost was at 30% clilorine dioxide substitution.16-0.with chlorine dioxide required in later stages to achieve the desired final brightnessfora wide range of degree of substitution (40-43). At 50% chlofine dioxide for the same sequence blcaching to 90 ISO brightness. This subject is discussed in greater detail in the chapter on chlorine dioxide bleaching (125). for bleaching soft™<?d"Й""**0* ™ ^S^T^T 3-8%. chloride ion was added. Assessment oi the impact of chlorine diox.rt&. for <DC)E ([Ш)bleacnjng.D+C. At 90% substitution the lowest cost was at 0. . cost was minimized at 0. as compared to mixtures (8). in the third and fifth stages of a (D+QEDEDsequence when mixlures.. To decrease the cost at 70-90% chlorine dlox e * substitution. . The minimum overall chemical requirement is achieved with 3050% chlorine dioxide substitution. before and after diermal aging. Figure 15 shows the even more dramatic advantage obtained when sequential treatment. r „. found minimum costs at 30% substitution independcnt()f kappa factor in the range 0. 45). ..18(47). . 27 0.15 0.3 •1. the addition of peroxide to the second extraction stage has helped to overcome this problem (48.17 0. 29.20 0.14 0.6 8.0 Kappa number 10' 30 50 70 90 'Sequence (C+D)EDED Some mills using only chlorine dioxide in the first stage have had difficulty in reaching high brightness (90 ISO).24 0.28 0.1 5.29 0.35 0.24 eucalyptus (4$) 0.8 K.9 6.28 0.31 0. kappa factor 0.24 0. As .30 0.1 3.2.1 4. 26.37 0.i number softwood (40) 6.17 0.17 0.2 Kappa number birch (42) 30 50 70 90 E. Chlorate is formed by reacUon of chlorite ion with hypochlorous acid. % of total equivalent chlorine 10' 30 50 70 90 10" 30 50 70 90 С 18.26 0.27 0. 3. kappa factor 0.18 0.8 8.2 Kappa number 10' 30 50 70 90 10' Optimal charge in first bleaching stage.4 - » B.42 A.7 continuous cooking (41) 5.27 0. 17. 2.34 0.8 5.16 0.1 4.26 0.32 Kappa number after extraction Total equivalent chlorine to reach 90% ISO brightness.25 0. 49).17 0.17 0. Optimized (OCfFOFD bleaching pulp to oj braft 9(УХ<ISO brightness.20 0.17 0.1 4.20 0.7 4.17 0.ipp.29 0. the chlorate ion formed when chlorine dioxide is used in bleaching represents a loss of active bleachjng chemical.16 0.27 0.26 0.4 5.15 0.Table l.9 5. 16.24 0.17 0. CK\ in ПгМbleaching stage.18 0.17 0.17 0.8 Kappa number softwood from modified oxygen-delIgnilied softwood (45) D.28 0.25 0.31 0.36 0.28 0.4 Chlorate formation described in Sect.14 0.17 0. 21 0. Chlorate formation decreases oxidizing chemical available for dclignification. and consistency. СЮаin first bleaching stage.21 0. in the interval before chlorine addition.15 0.15 0. the chlorate formed is decreased compared io mixtures or to (CD). kappa factor 0. 50-52). chloride ion concentration. In a (DC) treatment. pll. kappa stage. % of total equivalent chlorine 10' 30 50 70 Optimal charge in first bleaching stage. Total equivalent chlorine to reach 90% ISO brightness kappa factor 0. in pan accounting for the increased efficiency of this sequential treatment.17 0. there are other factors that also contribute to dclignification efficiency When chlorine dioxide is added first in sequential treatment. Optimized (DC)(E+Q)D bleaching of 12. chlorate formation can be decreased.17 0. chiorile undergoes acidic decomposition to give chlorine dioxide and chloride but no chlorate (16). particularly the mode of chlorine/chlorine dioxide addition.similiar results have been reported by others (42.By changing the process.0 kappa number oxygen -bleaebed eucalyptus pulp (33). Figure 16 illustrates this statement (30J.23 90 'Sequence (C+D)BDED .in a (CD) treatment. Conversely.however.22 0.25 0.32 Table 2. (DC). or a modest flowthrough tank.25 charge: kappa factor Chlorine dioxide 25-10096 of the charge: Temperature: Т . Typically. and chlorine addition followed by a 60">'nute.. The percentage chlorate formation is higher lor chlorine dioxide bleaching stages as discussed in Chap. 1094 consistency retention tower for the chlorine dioxide reaction.1 Flowsheets and equipment *de?** %tm^'* л'°Т. Gcrmgard has shown that. Process flowsheets and . 18 (58.. typically 3-494.hypochlorous acid is present when chlorite is formed by the reduction of chlorine dioxide and this leads to chlorate formation. an example of which is given in Fig. and chlorine dioxide solution is added using a mixer. a dilution step. At this time. 19 (58). to provide retention time. 4. This flowsheet is appropriate for 30-70% substitution with лj.cllC5: flowsheets (55-57) and another report of successful mill trials provides five examples of process flowsheets. delignilication efficicnc >' *™Ю™ «*« **■ "»■ total 50-60"C 0Й? "* Са UPIO5mln Total time: lind pll: Consistency: Tliis effect can also be seen bv plotting ' exttacted kappa number against'ehemical consumed as shown in Fig. chloline dioxide was added to the dispersion of chlorine gas in water immediately downstream of the chlorine disperscr 03).|dc igniflcations(agettSingchlorinedi *. Ыпcxtcmщ$м3ondcUgnification | efficiency. softwood pulp used had a kappa number 1.*.Mal3-5% consistency (54).one mill employed a 9-minutc.„—«-*»-. .. 17.-4«J____i_____л. The 20-60 min.-. that is. examples of flowsheets. 59). it can be seen that chlorate formation.15-0. the incrcmental chemical charge required lo achieve an incremental kappa number decrease bccomes greater. In a 1967. Although chlorate formation is strongly affected by process conditions. the curve flattens.5-3 of 26. Chloratc formed is a function of chlorine dioxide applied.the mixed suspension passes into an upflow tower and then to a washer. Medium-consistency (10-15%) chlorine dioxide deligniftcation is not widely practiced at present but is be:■ ing adopted more wider/. As more chemical is applied. Several recent mill reports pro- 4. IV 8 (105)..„-i______. 3-5% consistency tower. . is approximately 20% on a weight basis (13% on a mole basis). 4. An example of a medium-consistency flowsheet is shown in Fig. In a 1973 report on sequential (DC) treatment of softwood. 20 (60). Reports of mill implementation of chlo thai is. Гпеflowsheet must provide suitable process conditions for effective chlorine dioxide delignification.consis. When chlorine is also added and it is added after the chlorine dioxide. most delignilication stages using chlorine dioxide in combination with chiorine oralone are modified chlorination stages. a second miU employed two in-line mixers separated b a >' 3-mmute section of pipeline followed b a4 minute >' ^ ">wer. Trial in which hardwood pulp was delignified with chlorine dioxide/chlorine mixtures. All the data for (IK!) treatment from the reports cited above have heen plotted in Fig. the chemical becomes less and less rine dioxide delignification provide many effective and delignification efficiency dc- .maii portion of the chlorine dioxide (5ш%) being added with the chlorine to po vent viscosity loss. Unbleached pulp is diluted to a low consistency. (C+D) was at 15% substitution and <DC)wasar50% 3-4% substitution. expressed as a percentage of chlorine dioxide consumed. as delignilication is extended. oxidcnasa|)vadvbeendiscussed>toacer. those conditions fall in the following ranges: Total chemical 0. there are two mixers sepa rated by a section of pipe.2 Chemical charge Thejssueofh(JWmud)chemica|loш m. Conditions ллcin. an amount corresponding to the overall bleach plant as discussed in Section 3-3. The lime required for complete reaction of the chlorine dioxide is a function of temperature. It was found that a 5-minute . reacts with the pulp before chlorine is added and is more efficient than when chlorine and chlorine dioxide mixtures arc used. the curve flattens at a higher extracted kappa chemical to the range in which it is most number.the steep part of the curve. and kappa factor.3 Timing of chlorine dioxide addition Chlorine dioxide. Optimal chemical charge must be determined in the context of efficient. The logical extension of this finding is therefore that delignificalion efficiency is maximized when all the chlorine dioxide is consumed before the chlorine is added. added in advance of the chlorine. iiarly laboratory work. thai is. 4. on the liming of sequential delignificalion was done only at 20"C with hand mixing and chlorine dioxide was limited to 50% substitution. consistency. As the chlorine dioxide in the first stage creases. by Hatton (8). There is merit in limiting the charge of increases above 50%. temperature. it is recommended that a small amoimt of chlorine dioxide (5-10% of the total charge of equivalent chlorine) be added wilh the chlorine. Further discussion of this topic is found in Chap. it was shown that if 60 seconds or 5-minute chlorine dioxide reaction time atmore were used. incorporating a siirred tank reactor temperature was insufficient to give maximumat low consistency. where the combination of time. and amount of added chlorine dioxide lead to complete consumption of protective chlorine dioxide. in fact. 5 minutes gave greater delignification30 seconds was sufficient to obtain almost maximum delignification and higher brightness (H). was poorer at 15 minutes*chlorine. in the same study. To avoid viscosity loss. it was found that 60 seconds was required to obtain maximum delignification at 40"C . Hatton also used a(61). Ill 2 on reaction (27).chlorine dioxide reaction time at this With efficient laboratory mixing. In another Mudy. benefit. pulp viscosity suffered because all the chlorine dioxide 20"C. it was shown that at 30°G and with 30% substitution. However. delignification was not increased at 10had disappeared before it could be useful in protecting the pulp against minutes and. chlorina-lion (125). 4 Time and temperature Time and temperature are critical factors in determining the extent of consumption of chemical in delignification using chlorine dioxide. have fixed volume towers built originally for low consistency chlorination. If a longer retention time is required. the retention time is much lower m operation than originally designed. a slight increase can be achieved by increasing consistency although this strategy is very much limited by the "pumpability" of the pulp. the time required to achieve 95% consumption of chemical decreases rapidly as temperature is increased (64).for 30-50% substitution (62). no dilution water other than recycled first-stage filtrate is used lor dilution of the pulp leading into the delignification stage. the chemical applied is not totally consumed and residual chemical remains at the end of the stage. A higher delignification ratc occurs with pulp having a higher kappa number. The rate of delignification controls the consumption rate as discussed earlier in the section on kinetics. The retention time available in such a stage would typically have been designed for 30-60 minutes. In modern mills.This is clearly shown in Hg. with production rate increases over and above design as is often the case in mills. with or without chlorine. The principal means of overcoming limitalions in retention lime is to increase the temperature. As shown in Table 3. however. and with chlorine dioxide/chlorine mixtures in the range 3060% chlorine dioxide as compared to chlorine or chlorine dioxide alone. In many cases. the chlorine dioxide reaction time required for maximum delignification decreased to 20 seconds for the same conditions. it is important to note that if residual is consumed too rapidly shive removal in this stage may be adversely affected. Temperature is controlled by regulating the temperature of the incoming tinbleached pulp. Tsually the principal objective of the chlorine dioxide/chlorine delignification stage is to achieve a particular degree of delignification by consumption of a specified amount of chemical. usually by adjusting wash water temperature in the last stage of brown slock washing. the delignification rate doubles. In the same stud)' it was found that when the consistency was increased to 10%. with application of a higher concentration of chlorine dioxide and chlorine. If the time is too short or the lemperature too low. Most of the discussion in this section concerns this objective. but. where other sources of water are uscd. Increased dcligniflcation results if the chlorine dioxide is completely consumed before the chlorine is added.tcmperature control is possible by adjusting the . I lowever. The high dclignification efficiency is mainly due to the high efficiency of that phase of sequential delignification occurring immediately after chlorine is added (63)4. at higher temperature.19 (47). 21 for deligniJuration of 31 kappa number softwood kraft pulp at 30"C with a kappa factor of 0. mills delignifying with chlorine dioxide. For each I0°C increase in temperature. this same influenced by pll with delignification bestudy showed that. heat exThere is no evidence that temperature changers may be used to heat the chlorine alteration has any impact on the quality of dioxide solution.temperature of rhe dilution water. However. As a final note. pulp produced by chlorine dioxide deligControversy exists about the impact of nification. In Canada. it was found that deligni. near an end pH of creased as the temperature increased. Anbleaching stages. delignification efficiency dcnumber after extraction. Rapson studied the effect in the 30-70% range when the temperature of pH on chlorine dioxide/chlorine stages was increased from 20 to 50°(* at a 0. the as chlorine dioxide delignification temperaraw water temperature may change from 2 ture increases is shown in Table 4 (65). neither pulp vishigher temperature on delignification efficosity nor kappa number is adversely ciency. negative effect on temperature.155. fication with mixtures of chlorine and The карртnumber afer the extraction Stage . as evidenced by the kappa was 0. Some chlorine dioxide at 50-60% substitution was delignification stages are subjected to wide significantly less effective at 60°C than at fluctuations in temperature due to seasonal 30°C (61).20 and found that delignification was slightly kappa factor (64). showed the ar> residual in either chlorine dioxide (66) or sence of a temperature effect on dclignifichlorine delignification (6?). In 2 for both chlorine dioxide alone and a 70/ yet another study. cation of unbleached softwood kraft and oxygen-delignified softwood kraft using 4. It or 3°C in the winter to 20 or 25ЭСin the should be noted that the time to reach zero summer.30 chlorine dioxide/chlorine mixture (68). variations which provide large changes in Another example of decreased efficiency the raw water temperature. There is some controversy other study failed to demonstrate any effect about the optimal pli for chlorine dioxide on delignification efficiency for substitution delignification. To offset this rinc dioxide and chlorine.5 pH chlorine dioxide alone or a 50/50 mixture pH has a significant impact on most of chlorine dioxide and chlorine (9). A thorough study by Germgard affected by extending the retention time (3 5Лdiscussed previously in the section on wc|| beyond the lime needed to exhaust the delignification efficiency. Chlorine dioxide solution is typiresidual for a 100% chlorine dioxide treatcally applied at 5-10°C and is added to the ment is much greater than the time given system in sufficient volume to decrease the in Tabic 3 for sequential addition of chlotemperaturc several degrees. when the kappa factor ing greatest. sodium chlorate formation was significant but thereafter decreased sharply up to pH 4. However.iorjne are used.3 "natural pH" to 1.8 kappa number oxygendelignified eucalyptus pulp (70). and 24 kappa number southern pine (71).4 kappa number with chlorine dioxide indicated a steadily decreasing DED brightness with decreasing initial pH in the first D stage from 8. deligniflcation efficiency was not directly related to the formation of non-reactive oxidant (QOj' and ClOp as shown in Fig. 22 for chlorine dioxide alone. 33 kappa number softwood kraft (66).» valueconsistent with the minimum loss of chemical in the form of chlorate and chloriu * CT5jIhe kinetics of chlorine dioxide/chlorine damnification are affected by pH in a manncr c°«P"c«c<J ЬУthe chloride ion con<*nlra»°" *"* ***** ?**&a. in an identical manner to that found to occur in chlorine dioxide bleaching. . 13. Another study provided supporting evidence indicating that the maximum delignification efficiency and OH brightness occurred at a pH slightly below *. this minimum did not correspond precisely with the optimal pH for delignification which 2 _ .50 Mat all pi I levels. the delignification Rltc is ag-iin lowest at low chloride ion concentration but increases substantially with increasing chloride ion concentrations up to 0. The delignification rate decreases from pH 2 to pH 4 in the absence of chloride ion but increases from pH 2 to pH 4 in the presence of chloride ion (20).5-4 to 1. Two studies reported the optimal pi I to be in the range of approximately 3-4. In the oresaceы0. treatment of oxygen-delignifled kraft pulp of 22. In one study.onf increasing the pH from 2 leads to a steady increase in delignification rate. Above pH Ichlorite formation increased very dramatically. Several studies by various authors have shown chlorine dioxide delignification to be insensitive to pH from an end pH of less than 2 to an end pi I of 4. 16 kappa number eucalyptus (43).5 respectively (72).„ S()diumсЫогШ(5>в^ IncdeUgnificatton rate is higher at all pH levebbuladislinclminimumappearsalpH 3 (J8)wllcn^UKS of chlorine dioxide andci. . However.increased slightly as (he pH Increased to 4 and then increased dramatically with further increase in pH. for 28 kappa pine kraft pulp and 17 kappa oxygen-delignihcd pine kraft pulp (69). The minimum chemical loss in the form of chlorate and chlorite occurred at a pH between 3 and 4.5: this range corresponded to end pHvalu** ranging from approximately 2. At a pH below 2. and oxygen-delignified kraft pulp of kappa 13. oxygcn-dclignificd pulp because of incomplete washing. Until recently. 4. another early study provided evidence showing that increasing the pulp consistency from 0. when the chloride ion concentration is less than 0.5 M. and 50% chlorine dioxidc substitution (62).The traditional measurement. 30. Separate studies on the delignification of hardwood pulp using chlorine dioxide revealed that addition of chloride ion decreased the kappa number alter extraction and decreased chlorate formation (4$. relatively little information has been published for the case where chlorine dioxide or chlorine dioxide/chlorine is used in the first stage. An early report of mill trials provides evidence of the increased chemical consumption thai may result from carryover (54).End pH in [be range 2-4 has no affect on pulp viscosity (70.ty Bleached pulp darkens with age. there has been no standard proce«Hire for measurement of brightness stabil- .001 M. 1. Carryover can be determined by total organic carbon (T(X7) measurement or by oxidantconsimiption^orexample. It is possible ">accelerate aging by elevating the temperature and. w.permanganate consumed per volume of filtrate .. This "brightness reversion" is a function of chemical structures in the pulp. rccvdcRccvcIedfiltraic(rorn theсп1огшеdioxide/chlorine stage can also conf.6 Chloride ion Giverj the number of reactions in which chloride ion takes part as shown in Fig. Independen. Per Ш1oforganictarbon.020.„. Recent studies show that increases in consistency from low to medium to high increase delignilicaiion efficiency of 30-35 kappa softwood kraft and decreases chlor ate formation (75)..7 Consistency In the 1970s.l. Another repon provides numerous examples of superior delignification obtained at medium consistency as compared to low consistency lor scquential treatment of oxygen-delignified kraft pulp with L0.. 72). "saltcake losses. In another study it was found that only 0. studies of hardwood pulps have shown the significant benefit resulting Irom an increase in consistency from low (34%) to medium (10-14%) in delig nification efficiency for eucalyptus ox 43).01 M chloride was required in a chlorine dioxide delignification stage to increase final brightness in a D(liO)D bleaching sequence by 1% ISO.1 (72). ._ _ . for light-sensitive struct\ttCS.<i%-l0% in delignification of oxygcn-prcblcached softwood kraft pulp with chlorine dioxide had no impact on dcUgnification efficiency G5>. 66). However. it is not surprising that chloride ion con centration has a significant influence on delignification kinetics and efficiency. In the chloride ion concentration range 0. 51 Br." bascdonsodiumor conductivity measurementi8notappropriateinsystemscmplovingnUraU. -8 Carryover of dissolved organic material It is well-known that residual dissolved organic matter retained by unbleached or. extraction stage iillratcconsumes more chemical than docs ncyt^6firstStageШ[Га1еand black liquor con!rtimcsmorcchcmica| than does extracdelignification both decmjj/tlhcchcmical consumed by filtrate ^n-over in filtrate recycle svstems using . However. delignificalion efficiency is decreased by almost 50% (35). delignilicaiion studies ol high-consistency pulp (>35%) using gaseous chlorine dioxide and chlorine showed that an increase in consistency led to a dramalic increase in delignification efficiency (74). pulp quality _--«■ . 4... delignification efficiency is unchanged but.crfyj\ сЫогшсdioxide tionstagefihratc(ШQnand10()% 4 5. This is explained by reaction pathways and reaction intcrmediales which provide a sound theoretical basis for these empirical observations. consumes bleaching chemical in a chlorination stage. by exposing the pulp to light.ghtness stab.„mc chemical in a closed system. aspen (66).. that is. times.content is associated with this increase in Ity. it Is likely bleached sulfite pulp. On the other hand. It has been well-25. pulp does not change with increasing chlorine This finding has been confirmed more recently both in the laboratorydioxide substitution (32. Bven though (78) and on a mill scale (79). 78). When bleach- . a 5. has been reported to increase brightness stability. Although mechanical pulps arc light sensitive. of chlorine dioxide substitution is used in the bleaching of mixed Canadian hardwood species (25. 81). A significant decrease in extractives content canthat chlorine dioxide substitution has little be achieved when chlorine dioxide replaces chlorine (80). accomplished by increasing the temperature. 26.impact on brightness stability of softwood pulps hardwood pulps have been found to be more stable when a high degree(12). for accelerated aging. However. Poor brightness stability has been shownthere is some doubt arising from the lack of a to be directly related to the high extractives content of chlorine-standard test method for reversion. Once again. the viscosity of fully bleached pulp has been found to be unchanged as chlorine dioxide substitution increases from 10 to 50 to 90% as shown in Table 5 (82).2 Viscosity and pulp strength decrease in extractives Pulp viscosity increases dramatically with only 5-10% chlorine dioxide substitution and then decreases slightly as percent substitution increases further at a fixed chemical charge (Fig. 23) (25). and humidities have been used. Several authors have claimed that softwood pulps arc more heat stable when delignified The use of chlorine dioxide in place of chlorine for delignificationwith more chlorine dioxide and less chlorine (6. The decrease experienced in the mid-range of percent substitution is likely related to the greater delignification achieved and the comparatively lower post-extraction kappa number that results. a range of temperatures. Similarly. exposure brightness stability. accelerated aging of chemical pulps is principally For softwood pulps the situation is less clear. there are also data which known for many years that substantial replacement of chlorine byshow that the brightness stability of softwood chlorine dioxide increases the brightness stability of sulfite pulp (25). The relationship between viscosity and strength is complex as discussed in other chapters of this book (125). 51. Addition of chlorine dioxide in advance of chlorine gives the highest viscosity at a given kappa number (31). Pulp viscosity is also sensitive to the mode of addition of chlorine dioxide and chlorine. 78) and birch (42). . chlorine dioxide substi*■. Data listed in Table 6 show that. The Arrhenius activation energy for chlorine dioxide reaction with panicles. A rate equation for panicle removal during chlnrine dioxide delignificaiion is given below (85): -d(l*"ticles)= к[CIO/" [particles]' dt where panic|c. that is.:. 5. the cellulose degree of polymerization may decrease and with it viscosity-. tution in the 1 (МЮ% range has no influence on tear (82). An increase in substitution from 10% to 30. Increasing the chlorine dioxide substitution from 30 to 100 % and decreasing the chlorination factor led to a similar increase in tear at a given tensile index for an oxygen4lelignified softwood krafl pulp (84). cornpares to a value of 60 kj/mol for reaction with lignin as measured by kappa number decrease.„i/w«w i_ • . A comparison of pulp strength data. and so this is a lirst-order equation. 83).i. It is now generally accepted that strength comparisons can best be made by comparing tear strength at a given tensile strength. This is distinctly different from the concentration of lignin which decreases very rapidly initially and then more slowly. Beyond a critical point a decrease in viscosity lead> to a decrease in sheet strength. 35 ± 5 kj/mol (85). but chlorine dioxide substitution has no effect (82). particularly those in early studies. Two mill studies have shown that significant increases in tear occur at a given tensile with increasing chlorine dioxide substitution. 40.t=shivcsknolsшь^ .although oxygen delignifieation leads to a lower tear at a given tensile. or 50% increased the tear strength of softwood krafl pulp by 10% (55). It should also be noted that oxygen delignifieation increases the beating time required to achieve a given tensile strength. Thus. on tear at a given tensile 01.ing chemicals attack the cellulose fraction in pulp. . is complicated by the lack of standard testing procedures. Other laboratory studies have shown that 10-90% chlorine dioxide substitution has no effect on strength. Thus. increasing substitution of chlorine dioxide for chlorine logically should reduce degradation of the cellulose.3 Cleanliness Removal of contaminating panicles is animportant function of bleaching. lower temperature in a . particle removal is favored by prolonged retention at a given concentration of chlorine dioxide. к = rate constant <varjcsасажИtoIidc) The panicle concentration decreases lin early with time. Because chlorine dioxide is known to be less damaging to cellulose than chlorine. chlorine dioxide alone is unilormly superior to chlorine alone in the firsi stage of a five-stage sequence over a range of targeted values. Substitution of chlorine dioxide for chlorine decreases the organochlorine content in pulp (80) as shown in Table 7 (90). a similar resuit is seen for a given consumption of bleaching chemical. As shown in Kg.g. The organochlorine content of pulp can be subdivided into water extractable and carbon adsorbable (AOX). Chlorine dioxide alone has been found in several laboratory studies to be highly effective in removing particles (85. 50. die bleaching of sulfite pulp to produce a dissolving grade with low extractive content was one of the first uses of chlorine dioxide in cornmcrcial bleaching. 24.. 80. the most important of which may be the extent of bleaching in a particular stage or sequence. 5. Lower temperatures therefore favor particle eliinination. several recent mill studies have shown that no change in pulp cleanliness occurs when chlorine dioxide substitution is altered over a wide range (13. In Fig.chlorine dioxide dclignification stage decreases the rate of deligniflcation and the rate of chlorine dioxide disappearance while the rate of particle elimination is reduced to a much lesser degree. partides may not be immediately eliminated in a particular stage but may be made more susceptible to removal in a later Mage.55-57). 25. 79.5% on pulp to 0. sulfite softwood or (craft birch) With chlorine dioxide rather than chlorine leads to decreases in the extractives content of the final product. further. 87). To complement information in the literature about the effectiveness of combinations of chlorine and chlorine dioxide in removing particles.5 to 0. An increase in chlorine dioxide substitution from 0 to HH)% decreased the ex«actives content of a sulfite pulp by 50% (from 1. The extractives content of fully bleached birch kraft pulp decreased from 0. and inextractablc fractions (88>89).4% on increasing the chlorine dioxide substitution from 0 to 100% and the chlorine content of the extractives decreased from 25% to approximatch/ 1% (42). The first chemical applied in a bleaching stage is consumed mainly by lignin rather than by particles and so extended bleaching with greater addition of chemical leads to improved particle elimination. The substantial impact of chlorine dioxide substitution on extractives in sulfite pulp has been reported frequently (25. Another study Showed that 30. Indeed.4 Extractives and organochlorine It has long been known that the bleaching of pulp having a high extractives content {e. solvent cxtractable. 86). Particle removal in bleaching is dependent on many factors. .75% on pulp) after CEDED bleaching (25). and 70% substitution was decidedly inferior for wood dirt removal compared with 10 and 90% substitution (16). Figure 24 shows thai mixtures of chlorine dioxide and chlorine are not as effective as chlorine dioxide by itself. Effluent properties 6. the amount of chlorinated organic matter formation of chlorinated dioxins and furans during in the effluent decreases. VII 3 (125).15. profound affect on dioxin formation. This (34). Following the discovery. TCDF andTCDD arc no longer detectable in pulp 01-93)Because of their very low concentration and limited solubility in bleaching effluent. several mill studies have shown that. However. Non-detectable concentrations have been achieved at several different chlorine dioxide substitution levels: 57%(55). the formation of 2.) multiple was reduced to below approximately 0. In one such study involving the bleaching of softwood kraft in an 0(DQ(EO)HD sequence.3. at 52% substitution. 7. chlorinated dioxins and furans arc difficult to detect and measure with the result that little information about them has been forthcoming from laboratory studies.Q per kg in whitefish fillets (95). as chlo- .I" another mill study. extensive research was undertaken organic matter was measured asTOCI (total organochlorine) 10 discover means of avoiding their formation. Substitution Early research showed TOG! to be much less than AOX (91). 6. chapter on dioxins (Chap. the concentration of dioxins and furans decreased to less than 5 mgTF. VIII3) (125). However. Numerous recent mill studies of dioxin control techniques have been reported. as chlorine dioxide substitution increased to 100%. Another study showed that. and 70% (94). 8-tctrachiorodibenzop-dioxin (TCDD) and its ftiran analog (TCDF) was eliminated (91). As chlorine was replaced by chlorine dioxide and die chlorine (CI. chlorinated pulp bleaching. Similar studies have shown that a combination of low kappa factor and high percent substitution can be used to maintain the application of chlorine below the threshold which leads toTCDD formation (92). at substitution greater than 50-70%.5 Dioxins in pulp Numerous reports indicate that. In earlier publications. 5. This figure decreased to zero at 70% substitution (25). In 1983 Ciermgard showed that it was possible to estimate the organochlorine formed by multiplying the amount of chlorine (CI) in all the chlorine bleaching agents used in a sequence by a factor Мб). of chlorine dioxide for chlorine was found to have a but a more recent comparison showed contrary results (97). ТОЮand TCDF became nondetectable in the pulp (5$). chlorine dioxide substitution was increased incrementally until. It was established in 1989 that increased chlorine dioxide substitution is one means by which the formation of chlorinated dioxins and furans can be reduced. as chlorine dioxide i*. in the mid-1980s. 6. of [he substituted for chlorine.2 AOX and EOX The first measurement of the impact of chlorine dioxide substitution on formation of chlorinated organic material was made by Rapson in 1966:6% of the chlorine (CI) charged was found as organochlorine.rine dioxide substitution increases. 85% (57).1 Dioxins Substitution of chlorine dioxide for chlorine has a profound affect on the formation of chlorinated dioxins as is discussed in more detail in Chap.TCDO and TCDF became non-detectable at 85% substitution (57). this method is not as accurate andmuch more subject is covered in considerable detail in die complex than the AOX (adsorh-able organic halogen) technique. TCDD and TCDF concentrations in the effluent decrease. 98) but has been found to be lower in mill practice (99).In these experiments.13 ibr laboratory-bleached softwoods(96. the factor lias been found to be much lower (0. the pulp kappa n umber was 20 and kappa factor was 0.07 and 0. I jebergoti also . This factor varies between 0.In laboratory studies on hardwoods.18. 26 (91).Axcgard showed lhal AOX decreases as chlorine dioxide substitution increases and varies linearly with the elemental chlorine (CI) used in the first stage as shown in Fig. AOX can be calculated by multiplying the elemental chlorine applied by a factor of 0.13.05) (100).04-0. that is. AOX was lower than anticipated based on the linear relationship observed at lower substitution levels. Another important factor in AOX for- mation is the kappa factor. As shown in Fig.the amount of chlorinated organic matter lhal is extracted depends to a major extent on the solvent used. 10/). 83. total sequenceAOX formation decreases with decreasing kappa factor in Do (108). 102-107). FOX and EOCI are tests which depend on the isolation of a fraction of chlorinated organic compounds that are only slightly soluble in water. the amount of AOX formed is low and can be further decreased by modifying several process variables.65 kg of AOX/ADi (56). A mill trial showed that. FOX comprises less than 1-2% of the AOX in bleach plant effluent. . The chlorinated organic compounds of greatest concern are those which are lipophilic. 27 (97. at 100% substitution. AOX has been shown to decrease linearly with |X)Sl-oxygen kappa number in the 16 to 10 range for softwood kraft pulp from an extended dclignification batch process (108). 28. There are many other studies of the impact of chlorine dioxide substitution on AOX formation (71. those which arc sparingly soluble in water and highly soluble in fatty tissue such as found in fish. FOX (extraetable organic halogen) is a parameter that includes all chlorinated organic compounds which can be extracted from bleach plant effluent with an organic solvent.found a linear decrease in effluentAOX accompanied increasing chlorine dioxide substitution as shown in Fig. At very high levels of substitution. liOX is also referred to as EOCI (extraetable organochlorine). (ienerally. When only chlorine dioxide is used in bleaching. 75. a softwood kraft pulp (post-oxygen kappa number 20) bleached in a D(E+0)DED sequence produced 0. These values decrease by approximately 50% when substitution is increased to 100% (56). are not formed when chlorine dioxide alone is used. This characteristic maximum is decreased or vanishes if the order of chemical addition is changed from DC to CD (52) or if the chemical charge is adjusted to provide a constant kappa number after a (DC)E sequence (106. catechols. that is. 30.but when the level of chlorine dioxide substitution exceeded 40%. Substances with an octanol-water partition coefficient of greater than I05 (log Kfflv= 5) are generally thought of as being compounds of concern. However. 124). some compounds. = Rj = H). only 5 mg of equivalent trichlorobcnzene per metric ton of pulp was found (109). The maximum . particularly those which an* highly chlorinated. R(= OCHs. When chlorine dioxide is substituted for chlorine. ether extracts many compounds that are not lipophilic. the total amount of chlorinated phenols formed increases to a maximum at 50% substitution and then decreases rapidly to near zero at 100% substitution (31. R. as a polar solvent. cyclohexane ЮХ: (91)). guaiacols. Results from a mill trial show cyclohexane EOX values for 60% chlorine dioxide substitution in the range 5-20 grams per ton. Lxtraction with ether gives FOX values of 300-500 grams per ton of pulp for 10% chlorine dioxide substitution and less than 100 grams per ton of pulp at 90% substitution (31. 111. Formation is strongly dependent on the amount of molecular chlorine (Cip used. a distinct relationship between EOX and AOX has been demonstrated as is illustrated in Fig. vanillins. The most abundant ring type is guaiacol (Fig. It is general!)' recognized that non-polar solvents such as cyclohexane and heptane provide EOX values which are more meaningful. At 0% chlorine dioxide substitution. and syringois ait* formed from the lignin remaining after pulping and may have from one to five chlorine atoms per aromatic ring.represent a small fraction of EOX.6.112). 110. 29 (heptane EOX: (98).7 have been isolated from bleach plant effluent.3 Chlorophenols and other organo-chlorine compounds only a small traction of the EOX is highly lipophilic. greater than 5. In two separate laboratory studies. 34). Increased degree of chlorine substitution leads to increased toxicity and a tendency forbioaccurnulationsothittpolychlorinatcd aromatic compounds are of greater threat to the environment. 200 nig of equivalent trichlorobcnzene per metric ton of pulp was found. Lipophilic chlorinated organic substances. Chlorinated organic substances with a log К . Chlorine-substituted phenolic compounds. which have a potential to bio-accumulate. Several types of chlorophenolic compounds are generated when chlorine dioxide is used in combination with chlorine to delignily pulp. The concentration of chlorine in high molecular weight material decreases as chlorine dioxide substitution increases (see Chap. Chloroform formation decreases linearly with increasing substitution of chlorine dioxide for chlorine with one study recording less than 10 g/odt being formed when chlorine dioxide alone was used (117). There are numerous other examples of mills using chlorine dioxide alone and recording non-detectable highly chlorinated phenolics (115). the critical factor is the amount of chlorine used. Many mill studies on the effects of substantial chlorine dioxide substitution include acute toxicity^ measurements (55. 91. 12. and pentachlori-natccl phenolic compounds at0.56. A lower unbleached kappa number. 118-120). Very little literature is available on the formation of any other chlorinated compounds resulting from chlorine dioxide use in delignification. The type and amount of chlorinated phenolic compounds formed are also strongly dependent on mixing (113) and kappa factor.1 parts per billion (95). Decreased acute toxicity to Ceriodaphnia and minnows was dependent on the manner of addition of chlorine and chlorine dioxide (52). was not necessary to eliminate highly chlorinated phenolics (115). 6. These trends arc illustrated in Fig.and tetra-substitutcd compounds steadily decrease to near zero (52. tetra-. acute toxicity to rainbow trout was found to decrease to zero with increasing chlorine dioxide substitution up to 90% in laboratory bleaching (101).109). It has also been shown that poor mixing increases chloroform formation (112). It has been found that. 104.at 50% substitution is mainly the result of dramatic increases in the amount of mono-and dichloro compounds formed: at 90% substitution the amounts of tri. tri-. such as can be achieved by oxygen dclignifi-cation or extended delignificaiion. However. measurement of the biological effects was generally limited to acute toxicity and mutagenicity tests. VIII 1) <125). as chlorine dioxide substitution increases.4 Biological effects Prior to approximate!)' 1985. Several reviews and laboratory studies of that time concluded that acute toxicity tended to decrease as chlorine dioxide was substituted for chlorine in (he delignification of pulp (1012.and penta-chlorinated phenolic compounds are non-detectable (116). as with chlorophenolic formation. Again. 83) and other biological tests (121). 31 for the formation of chloroguaicols (109). the amounts of chloroacetic acid 01) and tetrachlorofuranone formed (104) decrease. tetra. When chlorine dioxide alone is used. Later. it was reported that less than 1 g of chloroform/odt is formed at chlorine dioxide substitution greater than 75% (91). A compilation of twelve months of effluent data for a mill bleaching 30 kappa softwood kraft pulp to 90 brightness with a D(EOP) DED sequence showed non-detectable tri-. 118). the results are often inconclusive about the effect of chlorine dioxide substitution because the effluents for the base case (0% chlorine dioxide substitution) are . only dichlorocatcchol was found in the effluent (114). laboratory studies have also shown that effluents from chlorination stages are mutagenic and that chlorine dioxide substitution diminishes mutagenicity (f/. Mutagenicity disappears very rapidly when acidic chlorination effluents are neutralized. When unbleached softwood krafi pulp at 26 kappa number was bleached to 90 brightness with the sequence D(E+0)DFT>. In another study. 95). As 100% substitution is approached. BOD (Biochemical Oxygen Demand) decreases very slightly with chlorine dioxide substitution (II. The relatively high cost of bleaching chemicals is basically a function of percent chlorine dioxide and the price ratio of chlorine dioxide and chlorine as illustrated in Fig. or close to the detection limit. is pulp yield. In 1967. who con cluded that: ". 12. This is typical of many similar studies in which the lowest cost is obtained in the range of 30-50% substitution with the cost being relatively insensitive over this range. There arc many sources of information about color removal from laboratory (26. present insignificant risk to the environ ment from organochlorine compounds." 6. As reported in a the review by McDonough et at. one estimate is that a 1% decrease in color occurs for each 2% increase in percent substitution (II. (/22). More recent mill studies and mill surveys have examined 100% chlorine dioxide substitution and have reported a significant increase in bleaching chemical cost (45. 101).5 Other parameters: color. the results of ecosystem studies consistently indicate that increased chlorine dioxide substitution is one means of decreasing overall effects (98). In addition.104) and mill studies (56. the chemical cost increases sharply. chlorine dioxide systems or by treatment of effluent with sulfur dioxide. 32 01).. based on literature published up to that lime.101. chlorate. at 25% substitution. and COD Chlorine dioxide substitution decreases effluent color in proportion to the percent substitution..31...particularly after secondary treatment. chemical costs were decreased by 10% in one mill but were unchanged in another (54). Nonetheless. Another factor. 94. BOD. 12). Chlorate ion is a by-product of chlorine-dioxide delignification as was stated carl ier in this chapter in die discussion of delignification efficiency. The many reported studies of the envi ronmental impact of 100% chlorine diox ide bleaching sequences have been critically reviewed by Solomon et at. Chlorate is eliminated readily by the action of anaerobic bacteria in unacratcd pans of biological treatment 0-100% although there is considerable scatter in the data (I /. sometimes discussed in connection with chlorine dioxide substitution economics. This question was thoroughly examined by McDonough in a 1985 review where it was concluded that.. The cost of increasing chlorine dioxide substitution has been estimated from the results in numerous mill trials. 56. 7. 12). Effluent COD (Chemical Oxygen Demand) decreases approximately 10% as chlorine dioxide substitution increases from . Chlorate ion has been linked to toxic effects on brown algae at one mill in Sweden. Economic factors The greatest economic consideration in the use of chlorine dioxide in delignification is the chemical cost. 5 7). a more recent study reported that BOD changes very little up to 90% substitution and then declines slightly thereafter (101). 49. there are other significant sources of toxicity from krali mills which complicate test results. a trial on a hardwood kraft pulp indicated that increasing chlorine dioxide substitution to about 50% docs not influence cost (53). Лstudy made in 1973 showed that.initially non-toxic.mills bleaching with chlo rine dioxide (100% substitution) and employing secondary treatment and with dilution typical of North American mills. Another aspect of the economics.12). broadly speaking. Paul liarl and Ulf Germgard reviewed this chapter and provided helpful comments.substitution had no effect on pulp Yield 01. Substantial usage of chlorine dioxide in the first delignificalion stage was limited to 7 out of 51 bleach plants in Canada as late as I9«7. Acknowledgments Sterling Pulp Chemicals has provided financial support for chlorine dioxide bleaching research to the University of Toronto.but by 1994 virtually all bleach plants in Canada used at least 30% chlorine dioxide in the first stage and a dclig-nification stage using l(H)% chlorine dioxide accounted for over half of the total production of bleached krafi pulp (/23). . of bleaching chemical usage is the extent to which the pulp manufacturing industry adopts a particular chemical. oxygen bleaching is considered to be synonymous with oxygen delignification. This applies not only to chlorinated organic . for example. Furthermore. '•" Hls*ory The gestation period of commercial oxygen bleaching was a long one. lhe decrease mcolor' however. The main benefits of oxygen bleaching are environmental They derive from the feet that both the chemicals applied to the pulp and the materials removed from the pulp are compatible with the kraft chemical recovery system.and other pulp types. this rapid growth is likely to continue. as illustrated inFig. In this chapter. and color.l (/). As trends away from the use of chlorine and chlorine-containing compounds intensify. The industrial application of oxygen bleaching has expanded very rapidly in recent years.The first commercial system was not started Up until I9"0 and total world capacity in 1980 was only about 10. Introduction Oxygen delignification can be defined as the use of oxygen and alkali to remove a substantial fraction of the lignin in unbleached pulp. The process is usually conducted under pressure. bur also to other environmental parameters associated with bleach plant effluents.000 tons per day. By 1992 the number of operating systems stood at 155. will probably see application only in conjunction with oxygen. including BOD. decreasing the potential environmental impact of the bleach plant/The decrease 1s roughly proportional to the amount of delignification achieved in the oxygen stage.OVl'lOtd" * Oy e x gn Dl tl i l ai n ei l l fc to ** тут 1. Oxygen will be used to predelignify pulp to the point where the necessary o/one charge becomes small enough to be economical and selective. secondary fibcr. This enables the recycling of oxygen-stage effluent to the recovery sj stem by way of the brown stock wash ers.000 tons per «lay. by-products. O/one. Oxygen is usually applied to kraft wood pulps hut can also be used lor sulfite. though it possesses certain advantages over oxygen. and delignification is normally in the range of 35-50%. It began in 1867 with the patenting of an "Improvement in Bleaching of Paper Pulp'byjoy and 1.with a total capacityof about 85.nonwood. both technical and economic considerations will mandate the use of oxygen. COD. is larger than expected on the basis of the lignin re moved in the oxygen stage. In a typical cmbodiment of the pr<>ccss. -2 Process overview Figure 2 shows a simplified oxygen dclignification stage flowsheet. providing lor the high mass transfer rates made necessary by the low solubility of oxygen in water. In a 1915 patent Ci).The remaining obstacle to commercialization.oxygcn delignification remained uncommercialized. perhaps a wash press followed by a rotary drum washcr. leading to sharp decreases in pulp viscosity and strength. In I960 and 1961. Product gases and unrcacted oxygen are vented.high-shear mixers where oxygen gas is added together with additional steam. This is necessary to take advantage of the environmental benefits of the oxygen stage by capturing the lignin and other material dissolved here and returning it to the mill 1 . Nikitin and Akim (6) made a scries of contributions stemming from their interest in combining the delignification and viscosiiy control steps of dissolving pulp manufacture. the pulp is blown to a gas separator mounted on a blow tank.The pulp then goes to one or more medium consistency. usually sodium hydroxide or oxidized white liquor. The main reason was that. Despite these advances. and the pulp enters the first of two washing stages. Alkali. and the first commercial system was started up in South Africa in 1970 (9). is added to the pulp at the base of the feed tank and mixed with the pulp by the discharge pump . The mixers disperse the oxygen in the pulp.Thc use of two washing stages at this point refleets the importance of complete removal of material dissolved in the oxygen stage.Campbell (2) that involved passing heated air through an agitated pulp suspension. under conditions necessary to achieve appreciable delignification. Grangaard and Saunders (7) patented additional improvements aimed at making the process more commercially feasible. During the period from 1956 to I960. was soon overcome. cellulose was attacked and degraded. Mueller advanced the state of the art by providing lor pressurized operation and addition of an~alkaline earth metal hydrate. which then passes to an upflow pressurized reactor After a retention time of about one hour.This obstacle was finally removed when Robert and co-workers (8) discovered that addition of small amounts of magnesium salts sharply decreased the damage suffered by the polysaccharide components of the pulp during oxygen bleaching." Harris (4) and Marshall and Sankey <5) disclosed further improvements in 195^. unbleached pulp discharged from the rcpulper of the last brown stock washer passes to a steam mixer and then to a feed tank. AOX). -» ■■ . depending on equipment selec oxygen delignification tionandsitc(//. Also included under the heading of disadvantages arc increased steam costs.The use of oxidized white liquor as an alkali source requires additional causticizing.3 Advantages and disadvantages of 26 million. Additional savings result from a decrease in the caustic {i. i x 2 ing arc related to its beneficial effects on environmental parameters. capital cost is the most apparent. although there are also some operating cost advantages. it is worthwhile to review a number of its fundamental aspects. Recycling of oxygen-stage solids to the recovery systern increases recovery boiler loading and water. high (2<)-28%) consistency systems are common. added as steam and with chemicals.recovery system. The basic aspects of oxygen bleaching can be organized under the headings of chemisiry and process fundamentals. higher maintenance costs.Fundamental aspects of . This provides a logical framework for understanding the design and behavior of the process. the cost of a medium-consistency oxygen delignification system was estimated to be $14. in the course of one economic comparison of bleaching alternatives (JO). though few new ones arc being installed.An additional disadvantage is the tendency of oxygen bleaching to be nonselective at higher degrees of delignification. and an increase in overall process complexity. and the related concepts of . If this is chlorine or chlorine Uioxide. that is. sodium hydroxide) required in the first extraction stage. Any dissolved material thai escapes to the following bleaching stages becomes part of the effluent of those stages and is a potential environmental liability. Both decreases in chlorine dioxide require- rin edioxid e g enerates m uch lessA O X . it has been estimated that the installed cost of such a system with two postoxygen washing stages is in the range $ 1Э1.1 million.2). or ozone. In cither case. When the delignification stage oxidant is chlorine dioxide. Its chief disadvantages are capital costs and incrcascd demand on the mill recovery system. chemical oxygen demand (COO). Of the disadvantages. chlorine dioxide. the oxygen stage leads to a major decrease in biochemical oxygen demand (BOD). More recently. I'urthcr savings result from a decrease in the chlorine dioxide charge needed for final bleaching stages. Both types of systems are considered in more detail in a later section of this chapter.The main distinguishing characteristic of the high-consistency system is the presence of a dewatering device to produce high consislency and a down flow reactor. The most obvious beneficial effect of installing an oxygen pre-delignification stage in a chlorine compound-based bleach plant is a decreased need for oxidizing chemical in the delignifying part of the bleaching sequence.Anirthcrdrawbackis The chief advantages of oxygen bleachthe possibility of overloading systems in the chemical recovery area of the mill. Important chemical considerations include the cbemislry of oxygen. the effect is smaller because chlothe chemical cost saving is greater. Lower chemical cost results from the decreased requirement for delignifying oxidizing chemical (chlorine. bu t ОХудвПdelignification Before considering equipment eonfigurations and process variable effects in oxygen bleaching. and color.e. reactions of lignin and earbohydrales. increases evaporator loading (7. and for predicting the effects of changes in hardware or mode of operation. and oxidized while liquor usually provides the necessary alkali for the oxygen stage at low cost. there is a corresponding decrease in chlorinated organic byproducts in the bleach plant effluent (as measured by adsorbable organic halide. In 1987. In addition to the medium (10-14%) consistency-typc systems. lor example). because oxygen is less expensive. not more than about 50%.This is not a prob1cm if a suitable carbohydrate protector is used and the degree of delignification is not loo high. menl translate into the additional advantage of allowing a smaller chlorine dioxide generaior to be used. All three topics have been reviewed by Robert and dc Choudcns(20). both it and its anion are present . And Grat/1 (22).Carbohydrate reactions are the sub» ject of papers by Theander (18) and Sjosirom (19). are unpaired. Ог.each can readily acquire another electron to form. have been the focus of reviews by Gicrcrandlmsgard (75) and Grat/1 (76). HOO". Q£~>and the hydroperoxide anion. Although less reactive than other free radicals. ciently stable to form in appreciable Oxygen chemistry Oxygen is an unusual molecule in that its normal (lowest energy) configuration is the triplet state. Figure 3 illustrates the process.S) has reviewed the inorganic chemistry of dissolved oxygen and the various radicals and other specics derived from it.A more general review by C*«ierer(77) also discusses relevant lignin reactions. Both the superoxide anion and the hydroperoxy radical have higher oxidation potential (affinity for electrons) than their parent oxygen. In other words. Each of these electrons therefore has an affinity lor other electrons of opposite spin.GratzlfV. 2.selectivity and proleclors. the anion remains uncombined under the alkaline conditions of oxygen bleaching.8). Reactions of lignin. respectively. the dianion is probably not suffi amounts. the peroxide dianion. heal effects.In principle. In reality. inferred from the results of model compound studies. oxygen is a free radical. It contains two electrons that. because this radical is a weak acid (pK =4.8). The result is the initial step in a four-step process in which oxygen is reduced to water and the substraie is oxidized. Because hydrogen peroxide is a weak acid (pK.= 11. because of their parallel spin. which can combine with a hydrogen ion to form the hydropcroxy radical IlOO* However. therefore. called the supcroxide anion. The product of the first step is a negatively charged ion. and reaction products. The hydroperoxide anion is the dissociated form of hydrogen peroxide. Hisc (21).. it shares their tendency to react wicli appropriate substrates at regions of high electron density. Singh (14) has presented a comprehensive overview of the chemistry of the various radical species likely to play a role in oxygen bleaching.1 Chemistry Several reviews of the chemistry of oxygen dcUgnification that have appeared in the literature may be consulted for more detailed information. chemical consumption. Features of the process to be considered are rates of chemical reaction and mass transfer. the substrate is activated by providing alkaline conditions minated bv coupling reactions. An alhydroxyl radical is also a weak acid tcrnate pathway for initiation of the radical (pKa=l 1. reaction hydroxy! radicals in this process is partial2).9) that exists in equilibrium with criain reaction is abstraction of a hydrogen its anion. . Finally. . Oxygen in its normal state is a weak oxidizing agent. •(>. forming superoxide anion and a ide ion and a hydroxyl radical.enam|ш jcгаШо|(<Jformaxv lulose as well as ligninЛfinal point worth ^^ ^ ш аЫхгАаah notmg about F. the indiscriminately reactive species that are believed to be the source of much of the damage to cellulose that occurs during oxygen bleaching.' The complex oxidation processes dial occur in the oxygen bleaching reactor inelude radical chain reactions involving a variety of organic species derived from both lignin and carbohydrates. .in appreciable amounts under oxygen to ionize free phenolic hydroxy! groups in bleaching conditions. 4. Propagation of the chain reaction occurs larly significant because they are extremely byrcactioilsSlicha5thconc between oxyreactive and indiscriminate. the hydroxy! radical atomfromanunionized phenolic group or can acquire another electron to form byothcr functional group to give the corredroxide ion or water. such as the hvdrotnoxv radical. reaction I). . is the involvement of trace quantities of transition metals. An electron is ab ide accepts an electron to form a hydroxstractcd. attacking ecl.The phenoxy radical (Fig. HO". in some of these. MlШЫ complication. management of free transition metals concentration is an important constdcration in achieving selectivity in oxygen bleaching. acting as catalysts for peroxide decomposition.The resulting anionic The third step in the stepwise reduction sites are electron-rich and therefore vulnerof oxygen occurs when hydrogen peroxable to attack by oxygen.This catalytic decomposition forms hydroxy! radicals. so it is necessary to promote its reaction either by raising the temperature or by providing a reactive substrafe. 4. ' . Consequently. . Figure 4 shows likely initiation. In oxygen bleaching. the residual Hgnin. entirely new spe..es shown ^. HOOOvand singlet oxygen.^ ?and4Thechamfc^ (13). 3 is that a variety of rcacdalomюсшеаncwmc tions may occur between the spcc. propagation. The occurrence of sponUjng organic radical (Fig.g. and one oi great Significance. and tcrmination steps.* e cies are formed.. О •. More recently. this may be molecular oxygen (which is thereby transformed to the superoxide radical anion) or one of die many other radical species prcscnt. Free phenolic hydroxy 1 groups play a key role. explains why strongly alkaline conditions are needed to achieve appreciable deUgniflcation rates. The initial step is conversion of the ionized phenolic group to a phenoxy radical by the lo *s of a single electron to a suitable acceptor. The review articles already mentioned are a good source of information on these studies. studies of the effects of the process on structural features of both residual and dissolved lignins have led to increased understanding of the chemical and physical aspects of the mechanism (25).Lignin reactions Two general approaches have been used lo learn about the mechanisms of lignin removal during oxygen bleaching: Model compound studies beginning in the 1960s (23) and continuing up to the time of writing (24) have provided considerable insight. When ionized by addition of alkali.The resulting phenoxy . This.they furnish the high electron density that is needed to initiate reaction with the relalively weakly oxidizing molecular oxygen.A brief summary of the major findings is presented here. together with the weakly acidic nature of phenolic hydroxy! groups. The others correspond to introduction of hydrophilic groups. the superoxide anion radical. .P^Kminones. amltnc formation of muconic acid and Other acidic Structures that ionize and impart solubilirj to the lignin. depending on the location of the hydroperoxy function. . . a ring carbon conjugated with it. _ . or the hvdroneroxv . or at the frcarbon atom of the side chain. .. illustrated in Fig. Botnnpcsofreactions may|>e^^^ toenhance the solubility (>fthc. in part/as the corresponding anion.„ . lead to the formation of oxirane.. 1-igure 5 illustrates these steps.These reactions. Fi ure S 7 illustrates these mules. Hydrogen pcroxide is present as a result of oxygen . . andf r °b "■?"*■ ' hy^°per°^dcS roorth Md °..This may be the carbonyl carbon of the quinonc mcthide.. These re «СОЧИМlead to opening of the quinone ring. radical.radical is a resonance hybrid of structures in which the odd electron formally resides at the phenolic oxygen. reactions of unionized hydroperoxides. All of these positions are therefore potcntial sites for the next step of the reaction..-. 6. ej* In the alkaline environment of the oxvgen bleaching process. which can subsequently undergo an intramolecular nucleophilic reaction at an adjacent site. and сагbonyl structures. . conversion to a hydroperoxide.muconic acid. . j_ .. impartmgроЫcharacrcr.. The last corresponds to breakage of a bond joining two lignin monomeric units and therefore leads to lignin fragmentation.■■■ Pathwavs are also available lor similar .. jninlhca|ka|mcmcdium TV*.. the hydroperoxide intermediate exists. or an adjacent side chain carbon. 1 he resulting 4«in°nesaresusceptible to nucleophilic auackb the >' ">'drogen peroxide anion..L I -I *_I . at one of several different carbon atoms in the aromatic ring. .The other reactant can be molecular oxygen. capable of undergoing coupling reactions to form new carbon-carbon bonds between lignin units. A final point concerning the chemistry of lignin reactions during oxygen bleaching concerns the role of covalent linkages between lignin and one or more of the car- . The phenoxy radicals formed at an early stage of the process are. although there is some evidence for their occurrence (26). Studies employing pulp or lignin instead of model compounds have. These condensation reactions are obviously undesirable because they increase the molecular si/e of the lignin and thereby decrease its solubility. shows that additional phenolic hydroxyl groups and unsaturation in the side chain are both structural features thai ennance reactivity. Both observations are consistent with the results of earlier model studies. it was confirmed that the number of phenolic hydroxyl groups in the pulp lignin decreased and the number of carboxylie acid groups increased.reduction. as described above. That these include catechols. Pulp lignin structural studies have also shown that dipheny I methane-type condensed structures are particularly resistant to oxygen bleaching. 6 and 7. CJellcrstcdt and co-workers (28) investigated the structure of the lignin in oxygen-bleached pulp and coneluded that oxygen bleaching reduces the content of free phenolic units. thc condensed units are resistant to further reaction.confirmed the conclusions drawn from the model studies. and decomposition or hydrolysis of organic liydropcroxide intermediates. Лmethoxyl substitucnt in the aromatic ring also increases the reaclion rate.however.The relative rates of the various possible reactions have been the subject of a series of studies by I.junggren and Johansson (27). However. and some may react so slowly as to be considered stable. On the other hand. in principle. Because of the restricted mobility of the phenoxy radicals in lignin. to a large extent . In contrast. Certain structures known to be present in appreciable amounts in kraft residual lignin are very reactive. In a later study (29). it is unlikely that these reactions occur extensively during oxygen bleaching. the lignin remaining in the pulp after oxygen bleaching was enriched in biphcnyl-type condensed structures and p-hydroxyphenyl-typc lignin units. Residual lignin may contain many types of structural units that have free phenolic hydroxyl groups. and enol ethers. ail of which are subject to the reliction types depicted in Figs. stilbenes. not all react at the same rate. a carbonyl group in the a-position of the side chain or an inlerunit linkage at the 5-position of the aromatic ring decreases the rate. Furthermore. The уconcluded that cleavage of a bond between xylan and lig-nin would allow more extensive oxygen delignificaiion. Carbohydrate reactions .bohydraie components of the fiber wall. and that it was extensively degraded. who found thai a ligniii-carbohydraie complex extracted from pulp after oxygen dclignification contained about half of the residual lignin. This topic has been studied byTaneda and co-workers 00). 1'or this reason. breaking the glycosidic linkage joining the affected unit to the rest of the cellulose chain. Although both types may occur during oxygen bleaching. Traces of metals that are unavoidably present in unbleached pulps 02) promote random chain cleavage. The initially formed carbonyW^ontaining unit does not necessarily have to react as de- . oxygen-based radicals. random chain cleavage is the more significant. if allowed to proceed far enough. Reactions that degrade cellulose can be divided into two categories: random chain cleavage. and copper are particularly troublesome. delignifieation is usually limited to removal of no more than about half of the lignin in the pulp entering the oxygen stage. manganese. The ionized enol form of the resulting carbonyl-containing unit then undergoes a beta-elimination reaction. which may occur at any point along the chainlike molecule." by which units on the end of the chain are attacked and successively removed 01). They catalyze the formation of reactive. such as hydroxy! radicals. Transition metals such as iron. As illustrated in Fig. and endwise "peeling. The associated decrease in the average length of the cellulose chains manifests itself as a decrease in pulp viscosity and.Carbohydrates are attacked during the course of oxygen dclignification toagreater extent than during chlorination and alkali extraction.ihe initial step in the chain cleavage process involves oxidation of a hydroxy! group to a carbonyl group. as a decrease in pulp strength.8 (22). ultimately leading to chain breakage at the point of attack. that randomly attack the cellulose chain. In the oxygen bleaching of kraft pulp.The dis-covery <>l its effectiveness in 1963 by Robert and co-workers (8) provided a great impetus for the development of oxygen bleaching. the process is self propagating and can.These compounds are called carbohydrate protectors.scribed above to break the cellulose chain.For an end unit to be susceptible to removal by the peeling reaction. The protector of greatest commercial importance is the magnesium ion. li is normally applied at levels as low as 0. One approach for dealing with transition metals is to remove them by acid washing before the oxygen stage. Selectivity and protectors Selectivity can be loosely defined as the ratio of attack on lignin to attack on carbohydrate. yield loss is generally nol a serious problem.1 % Mg'2 on oven dry pulp. This is prevented by the incurrence of a competing reaction termed the stopping reaction that converts the end unit to one that does nol contain a carbonyl group. one of the most important is die transition metal content of the pulp. Epsom sail.copper. one of which is a reducing end group. peeling can become a problem if random chain cleavage is excessive because even' chain breakage creates two new chain ends. because of their long previous exposure to strongly alkaline conditions in the digester. in principlc. Because occurrence of the reaction leaves behind a new end unit containing a carbonyl group (a "reducing end group'). another is to add compounds to the pulp that inhibit carbohydrate degradation . It is believed to function by precipitating as magnesium hydroxide. In neither case is the cellulose chain broken. A competing reaction occurs when oxygen attacks its ionized keto form. Of the factors governing selectivity in oxygen bleaching. because these metals catalyze the generation of harmful radical species. for two reasons: Kraft pulps. and manganese.it must contain a carbonyl group.The other reason is thai oxygen itself converts reducing end groups to the stable oxidized form (32). contain very few end units that have nol already been converted юthe stable form by the stopping reaction. forming a cyclic carboxylic acid or an open-chain structure containing two carboxylic acid groups.Figure У(22) illustrates these reactions.making them unavailable for catalysis of peroxide decomposition (33) or by forming complexes with them (34). which adsorbs the metal ions. Figure 10(35) Illustrates the effectiveness of magnesium in preserving pulp viscosity . the peeling reaction. all of which have this effect. Most pulps contain appreciable quantities of iron. Since then a considerable number of compounds have been found effective. but none is as economical as magnesium sulfate or its hepialiydrate. is usually of less importance in oxygen bleaching than random chain cleavage. However. It is affected by the choice of process conditions and by the presence of pulp contaminants. The reaction that causes yield loss in alkaline media.continuc until all the cellulose dissolved.05-0. However.The mechanism of the effect may fundamental reasons for these effects arc probably related to theinvolve acceleration of lignin fragmentation multiplicity of reactive oxygen-containing species present and theirbecause of nitration of lignin monomeric different reactivities with the variety of lignin and carbohydrate structuresunits. selectivity usually process has been intensively studied and remains nearly constant until about 50% of the lignin has been removed developed by Samuelson and co-workers (40. 44). have been specifically studied from this cleavage of the intermediate ether linkages viewpoint by measuring their reaction rates with compounds(42). In describing and predicting the rate of the overall process. These include metals build-up and increased gaseous nitrogen oxide emissions. Hydrogen peroxide addition can enhance selectivity (3?) pilot plant stage (45) but has not been commercialized.even when the oxygen the corresponding conventionally bleached pulp. it is important to distinguish between physical and chemical phenomena. chlorine dioxide. although its viscosity is stage eonditons are mild. 2. Hydroxy! radicals. Physical factors govern the movement of the reacting species within . 10 that dioxide prctreatmeni is a development worth selectivity does not remain constant as the kappa number of the pulp is noting.ТЬсratio of thefragmentation of lignin by nitrogen dioxide lignin rate constant to that for carbohydrate was observed to lie between(4$. nitrogen dioxide in the presence of oxygen When delignification is no greater than 45-50%. Thebenefits. The process (Prenox") reached the 5 and 6. In virtually all of the existing 41).cause it favorably affects the composition of the reactive species mixture present. the target degree of delignification is 60% or less. Another likely factor is oxidative representative of lignin and carbohydrate structures (Зф. from the pulp after which it deteriorates. Selectivity may be different for different process types and is affectedconsiderably enhancing pollution abatement by process variables. Among the questions raised presumably bein assessing its viability are problems that may result from the necessary recycling of effluent from the acidic nitrogen dioxide stage to the kraft chemical recovery system. the oxygen stage can be as high as 80%. as is more fully discussed later in this chapter. and acidic hydrogen peroxide.2 Reaction and mass transfer rates The rate of oxygen bleaching and its dependence on process variables are factors that determine the design and size of the equipment and the optimal choice of process conditions. this decreased to progressively lower levels. Treatment of unbleached kraft pulp with pulping systems. cither of which may determine the rate under a given set of conditions. Selectivity improvement by nitrogen It is apparent from the nonlinear nature of the curves in Fig. Other pretreat-ments shown to favorably affect oxygen stage selectivity (46) include chlorine. and is typical of selectivities obtained in the bleaching of softwood kraft pulp. Pioneered by Ycthon (39). whieh are among the least selectiveseparated from their neighboring units by of the reactive species present. the resulting pulp may makes a following oxygen stage more be fully bleached to give a final product of strength equivalent to that of selective and efficient. Ugnin removal in usual!)' slightly lower. Such nitrated units are more readily available to them. Carryover of black liquor from the brown stock washers into the oxygen stage adversely affects selectivity (38). whilcchcmical factors govern the rate at which die pulp and bleaching chemicals react with one another once they are in contact (chemical kineiics).red amount ol heat m a steam mix andret chn hotfillratemu8tthet€ " ^ f ' forcЫ ' considered. the extent of reaction is limited by the availability ofalkali.000 . Depending on the conditions. Oxygen must cross the gas-liquid interface. „ „ .The heal of reaction is appreciable (53) and its removal mav).This can be done bv ensuring that the partial pressure of oxvgen in the gas phase and the gas-liquid intcffacial area are both sufficiently large.cаproblem in high-consistency.16% for hardwoods (54). Medium-consistency systems lend to consume slightly more alkali because the alkali concentration is lower at a given charge in these systems. which can often be made available at lower cost 23 eat .d.14. Depending on reactor type and conditions. at 27% consistency. diffuse through the liuukl film surrounding the fiber. In the limiting case.000 kj per metric ton for each unit0f ьЛррАnumber reduction (I I). hi practice. 24 times as much oxygen is consumed during bleaching at 15% consistency as can be present in the liquid phase at any one time and.and finally diffuse into the fiber wall before reacting..crin lhis leaml uscdtoevatoa * * « * * *f mix. 50 times as much..0' thcch. .cmicalгСЖ' **■""g*-moceisareavaUableforl>rc" d. chemical consumptlon an reactlon . high kappa-drop systems. the heat of reaction is 2400 kl/kg of material dissolved. alkali consumptions in this range are realized in mill-scale bleaching.The first type is removed within about the first 10 minutes in a rapid initial phase.the pulp mass (mass transfer). Chemical consumption The stoicbiometry of oxygen bleaching k sucn thai. pulp basis) is required for softwoods and about 0. effef+s " .The rates of removal of both types of lignin increase with increases in alkali concentration. Removal Of the second type proceeds at a rate that is proportional to the amount of lignin remaining at any given time. a viable process must facilitate the transfer of oxygen into and through the liquid phase.This means that the lignin content can be reduced to any desired level by allowing die reaction to continuc for a sufficiently long time. chemical requirements.13% NaOH (o. where mixing is so good that oxygen and alkaliarc equally available to all fibers.n ma v nrcac. especially in mediumand low<on*istcncy processes. i9). Mass transfer Mass transfer is an important consideration because most oxygen bleaching stages are three-phase systems. the rate ctmal * '° that. Kinetics Several studies have examined the rate dependence of delignification and carlxihydratc degradation reactions on reactant concentrations and temperature (5<K 50-The lignin in the pulp appears to be Of two differcnt types that differ with respect to the ease of their removal bv oxvgen. and types of reaction products formed have significant implications for the design and operation of oxygen bleaching systems. ^ effects. J"?"?** а1Ы' ^S?*"**!? *"*>bt' шlht'"■* 8*"5C'f° P™1* ш the « requ. oxygen partial pressure and temperature. The alkali is usually supplied in the form of oxidized white liquor. the rate of oxygen transport can limit the rate of the overall process because of the inherent slowness of the diffusion phenomena involved (47). The mass transfer problem is aggravated by the low solubility of oxygen in aqueous sodium hydroxide (48. In high-consistency systems. another places it at 12. about 0.'Ihey indica ethal f high-shear mixers are capable of ***&** ™y good mixing of alkali and ох >'*спwitn P"^ almcdiumconsistency. Obviously.According to one estimate.° products The requirements for application and release of heat. Typically. for each unit of kappa number decrease. die delignification rate may be determined by the intrinsic chemical reaction rate or by the rate of mass transfer of oxygen or alkali to the fiber.or S ff ^2>. the low water solubility of oxygen makes it necessary to 1) have a very large amount of water present 3 1 . Oxygen consumption is normally about 0. In particular. oxygen consumption increases sipnificantly. High-consistency systems constituted the majority of those installed during the first fifteen years of commercial oxygen bleaching. 3. Both types have advantages and continue to be available.The factor that most clearly distinguishes the available process types is pulp consistency. Because the last two products are combustible. Reaction products Reaction products include organic acids and carbon dioxide. Unoxidized white liquor can be used (38). and a few high-consistency systerns have been installed recently. beginning in 1970. small amounts of carbon monoxide. mass transfer considerations determine the configuration of the reactor and the provisions for efficiently contacting the pulp with oxygen.14% per unit of kappa number decrease for softwoods and 0. but there may be adverse effects on selectivity and rate <55)Afunoxidized white liquor is used. . it is necessary to control their concentrations in the gas phase ol high-con sistency reactors.than purchased sodium hydroxide. and traces of methanol. but most systems being in stalled currently are of the medium-consistency type.Medium-consistency processes Asalrcady mentioned. Oxygen consumption in high-consistency systems is slightly higher than in mediumconsistency systems because of losses due to reactor venting and cntrainment with the pulp leaving the reactor.16% lor hardwoods. This can be accomplished by continuously bleeding gas from the reactor or circulating it through a catalytic converter. the relevant reaction kinetics determine the size of the reactor. Processes and equipment The factors discussed above influence equipment design for oxygen delignification.56).Typical operating data ranges for the two types are given in Table 1 (54. Sunds Dclibrator (61. preheated in a low-pressure steam mixer.59).is equipped with a rotary div tributor to prevent channeling and in othcrs is simply conical in shape. Partially washed brown stock is fed to a washer and washed with filtrate from the post-oxygen washer. Channeling tendency increases . A typical system is represented by the flowsheet in Fig. 11 (54). the likelihood of channeling. and fed to the high-shear.This generally means providing a very large interfacial area between die liquid and gas phases. This problem was solved when medium-consistency mixing technology became available in the late 1970s.62). the pulp is transferred to a reactor. and oxygen gas passes to the bottom of the upflow reactor which.this was the only feasible method. When oxygen bleaching was first lntroduced.The new high-shear mixing devices make it possible to efficiently disperse oxygen as very small bubbles in 10-14% consistency pulp. Kamyr (58. The pulp from the washer is charged with caustic (XaOH) or oxidized white liquor.to dissolve all the oxygen needed tor dclignification. One way of doing this is to dewater the pulp to high (20-27%) consistency and fluff it to separate the fibers before bleaching to create a dispersion of fibers in a continuous gas phase. where the delignification reaction is allowed to continue. medium-consistency mixer by a medium consistency pump. After the oxygen is dispersed. or 2) use a smaller amount of water and provide conditions that facilitate continual replacement of the dissolved oxygen by oxygen from the gas phase.lmpco (60). the reactor is designed with a higher aspect ratio than is typical of other types of bleaching towers to mmimi/. Oxygen is added direetly to the mixer from which the mixture of pulp. in some designs. An advantage of this approach is that no special dewatering equipment is needed before the oxygen stage. and Kauma Repola (63). Processes operating in the 10-14% Gen sistency range have been described by. Operating at medium consistency was not feasible because of the unavailability of equipment for Intimately mixing a very-large volume of gas with the pulp suspension (57).The dispersion is relatively stable and the buoyancy imparted by the trapped oxygen eliminates any tendency for bed compaction. among others. alkali. In either case. Also. strucledot . The M0D0-CIL process (65. The washed stock is diluted.Itisthenblown to a vertical pipe on top of the blow tank that is equipped with an induced drali Ian to purge the tank of entrained gases. To maximize environmental benefits and cost savings.t. ' „. onlv pressure supplied is the hydrostatic ^ emcdbvfh.l lhanlhcsccond f f^ugh normally designed lor pressur. . These are sometimes re. usually 4060%._ . .75 rn in diameter and 27.. pariicularly in medium-consistency systems. medium-consistency systems (of. the gasdispersing ability of the mixer decreases and channeling becomes more likely as the volu- metric fraction of gas in the pulp suspension is increased. clad stainless steel.. ' . —.•. where it is further washed.. . v.9 m high and is designed for 4S minutes* retention at a production rate of 320 tons per day. Stock from the blow tank passes to a pressure washer. . . From there it goes to a multi-stage bleach plant. 12).Л161.zed °?*™*0а>mt4"umсо™^псУ■»* terns incorporating open towers are also .if the consistency decreases below 10%. many of the existing commercial installations operate at high consistency. Post-oxygcn washer filtrate is used for stock dilution at the top of the lower to facilitate dischargc. which is mechanically assisted. . .66) uses a vertical cylindrical reactor designed to conlain a continuous pulp bed at a consistency of about 27% (Fig. • „ . In operation. co|umnmtnc ^^ Hdrosla[ictemsart. . . without adversely affecting pulp quality.. the amount of rccycled. steam.. as is the amount of water that must be heated to the reaction temperature. . Feeding is by a screw . where it is washed with filtrate from the screened pulp washer. fcrred to as hydrostatic svstems because the . . AtlL the . For these reasons. this system is typical in .aycrthn)llghwhich oxygen must diffuse to reach the fiber. Limitations on the degree to which all of the oxygen added can be dispersed as sufficiently small bubbles and coalescence of bubbles may contribute to a slowing of the reaction as the pulp progresses upward through the reaclor. and residual oxygen to be released In one example (60)._____._„ .. Keeping the consistency al or slightly above this value prevents bed compaction.potentially oxidizable dissolved organic material in the reactor is reduced. Attempts to increase delignification by increasing the oxygen charge may be counterproductive. . this system has allowed bleach plant production to be increased from 320 to 410 ions/day with no change in pulp strength and with improved final brightness and brightness stability. and sent to the screened pulp washer. screened.ng by a rotatingplow-typedischarger.' .so b|c |ignjtyinkraftpuips.^d 0n"cn Г С С Ш Г no 1 С edition of oxygen or alkali he tweensta Cs Tnclirslreactorma bc B " >' sma.). The two are similar.This has the two-fold cffcclofprovicUngяIiirgegas-liquid interfacialarcaandГс4|исш8rncthickness of the .They differ from the single-stage systems only by the addition of amixcranu stcondгеасГог * between the *'rslreaclora*1*-1tntD. . Although smallcr than most.|quid . Two general types are available: one originally marketed by Kamyr as the Sapoxal process and the other by M0D0ChcmcticsatidlmpcoasthcMoDo-CILsystern. however. the reactor is 2. * * being used (6?). which would otherwise reduce the free volume needed to serve as an oxygen reservoir. These considerations have led to the dcvelopmenl of two-stage. Achieving these leveb in practice is often difficult. . it is normally desirable to remove as much lignin in the oxygen stage as possible. The reaclor is designed to c.owunk. which allows entrained product gases. the only major difference being in the design of the reactor itself. Thc stock is blown to a blow tank through a separator. . top of the reactor the stock is diluted to 10% convener before being moved toward a central discharge hous. nornia||y foundщsulfircmj||sbulafea.„ other respects. .especially hardofde WQodsf<>тЫсч(degr£CS(j|bomъо%) 32 High-consistency processes one approach 10 sobing the three-phase masstransferргоЫстisioremovemostof thcfrc( u<mJd phase. „ operate at a pressure oM 20 kPa and is coni w л*. . A small gas phase stream is con- . Alkali. the pulp is diluted to 5% consistency with recycled oxygen-stage filtrate. which is controlled by changing the flow rate of the leaving pulp suspension. Discharge is accomplished by a rotating conical screw and agitating amis. usually in the form of oxidized white liquor. Oxygen is added at the top or bottom of the reactor to maintain the desired partial pressure. and steam are added and the pulp is transferred to the reactor by a thick-stock pump or plug screwfeeder Steam is added at the top of the reactor to maintain the desired temperature. Upon entering the realtor.feeder or thick-stock pump. the pulp passes through a fluffing mechanism and falls onto the bed. The system is exemplified by those in operation at Eddy Forest Products in Espanola. Figure 13 04) is a generic flowsheet of the process. which maintains a seal -against the reactor pressure. At the bottom of the reactor. and at the Franklin. mill of Union Camp Corporation (68).W'ashed unbleached pulp is dewatcred to a consistency of about 30% in a twin-roll press. Ontario (67). Virginia. A nuclear gauge senses the bed level. which tends to become more pronounced us the consistency is decreased.11) is similar to the MoDo-CIL reactor. 14 shows. and improved selectivity in the presence of appreciable amounts of black liquor solids. The possibility of ignition of combxistiblc gases is rendered insignificant by taking simple precautions.and high-consistency systems Both medium. for example. allowing it to be returned to the reactor to reduce oxygen consumption. greater ease of stock handling with medium-consistency mixing and pumping technology. 64. The Sapoxal reactor (9. 56.The trays. rupture discs. processes.54. The trend in new installations is strongly toward the medium-consistency process. The concentration of combustible gases is controlled by maintaining a continuous bleed stream.6% 10. the bleed stream is routed through a catalytic system for destruction of combustibles. Prior to 1983. Concentrations of carbon monoxide. Bed compaction. and oilier volatiles in the reactor are monitored and care is taken to ensure that volatile additives (i. medium-consistency systems accounted for 82% of installed capacity. Their relative merits have been evaluated by several authors (10. virtually all installations were of the high-consistency type. As of 1993. In some installations. Advantages claimed for operating at the lower consistencies that this arrangement permits are low bed combustibility and a lowering of the temperature rise associated with the reaction exotherm.The usual safety devices include an automatic relief valve. 13). As Fig. of 19 new systems started up or slated lor startup in the period I985-I989.all but one were medium-consistency. but since 1985 virtually all have l>een medium-consistency. within limits. rotate with the result that each compartment is emptied into the one below once per revolution.tinuously bled from the reactor to remove inert gases and combustible reaction products.and high-consistency oxygen dclignification systems should he considered for any new installation. Other advantages claimed lor the tray-type reactor are better control of retention time resulting from the absence of channeling and a more uniform gaseous atmosphere. some defoamers) do not enter the reactor with the pulp. According to a 1987 review. chemical consumption is higher than manifestation of the kinetics of the compoin the high-consistency systems and the nent chemical reactions and mass-transfer extent of delignification tends to be lower. Each tray is divided into twelve compartments by radial walls and each has a cutout that coincides with the cross section of the conipartments. but operates at somewhat lower consistencies (17-25%). 12. to keep it independent of the production rate. Kinetic studies such as those of . the reactor contains a series of trays arranged one above the other. Among the reasons that may be cited lor this trend are lower capital costs. hydrocarbons. is avoided because the bed depth is never greater than the distance between trays. and temperature-controlled quench showers. It is therefore possible to vary the retention time of the pulp by varying the rotation speed and. methanol.e.3 Comparison of medium. On the other hand. for which the reactor is specially designed. but not the walls.. 3. Then: is an initial rapid kappa number drop followed by a slower one. both process types should be considered for any new installaion.The high-consistency system still has its proponents. both of which are iirstorder rate processes . Laboratory data indicate no difference in selectivity at the same level of magnesium addition (74). the kappa number continues lo drop indefinite!)'. This is in contrast to tne normal observation that the process appears to stop when a limiting kappa amn her is reached. 1 lowever. as shown in Fig. Process Variables The response of an oxygen bleaching system to changes in process variables is a Olm andTeder (50) therefore provide a useful framework for rationalizing and predictUig these effects. This is interpreted as being caused by the presence of two types of lignin that diffcr in ease of removal. A consequence of the first-order nature of the delignification process is that. 15 (75). In summary. The two dclignification stages are directly paralleled by two corresponding cellulose depolymerization stages. to maintain the same level of preoxygen washing. High-consistency systems often provide substantially more delignification and the chemical consumption difference may be significant. as evidenced by the fact that 700. the decrease of kappa number with time exhibits two distinct stages.000 annual tons of high-consistency capacity was scheduled for startup in 1991 f 56лThose favoring the high-consistency process claim that the advantages of the medium-consistency approach are overstated. the point 4 1Timeand . 4. The latter behavior results when the alkali charge is exhausted. temperature At fixed alkali concentration. given enough alkali. the medium-consistency system re<mires an extra washer to replace the press. Figure 15 also shows that delignification is considerably accelcrated by temperature increases. The investment difference is mainly the result of a simpler feeding system for medium-consistency and elimination of a press upstream of the reactor. lessening the advantage. Ibis relationship tends to be moderately independent of process variables although selectivity does suffer if the temperature is increased beyond about 120°C «r if all of the alkali is consumed. 10 shows. about 4 atm.. As Fig.mieof . the parallel between delignification and carbohydrate degradation rates leads to a good correlation between viscosity and kappa number.2 Alkali charge Increasing the alkali concentration by increasing (he alkali charge at constant consistency substantially accelerates both delignification and cellulose degradation.of alkali exhaustion i. 43 Oxygen pressure . Hgurea 16 and 17 (74) illustrate this relationship. 4.nccffcclofoxygcnpressurcisgenerally smaUm comparison to the effects of alkali Beyond mirnmumv. .s reached much more rapidly at 130CC than 85°C. for economic reasons.Theareas affected include screening.downstream bleaching. 64. Good upstream washing is essential because black liquor solids consume oxy-K*:nandтлУadversely affect selectivity.brown stock washing.the kinetic studies already referred to show that oxvgen pressure has an appreciable effect on the observed reaction rates at con-stant alkali concentration.and posu>xygen wash ing efficiency have been the subject of sev eral studies (56. On the other hand. and the effects of each are dependent on the consistency of the oxy gen stage.P washing before and after the oxygen stage ' f a n oxygen stage is to be used for predelignificaiion. it is most logically intc «rated into the brown stock washing sys lem.4 Consistency Despitc its far-reaching equipment and process implications.lowering the consistency leads to a mod eraie slowing down of both the deligni-fication and carbohydrate degradation reactions as a result of the associated de crease in alkali concentration. The operation and. 76. stock preparation.alkali is normally prescnt in limited amounts. Oxygen-stage washer fil irate is used as wash liquor in the brown stock system and therefore finds its way back to the recovery system.the effect of consis-tency at fixed alkali charge is relatively small.charge and temperature. 77). in some cases. increases in oxygen prcssure have relatively little effect. The dissolved so|idsemtring the oxygen stage arise from two different sources. Effects ОПmill operation The effects of installing an oxygen dclignification stage extend far beyond the bleach plant.chemical and heat recover)'.Good downstream washing is necessary to fully realize the pollution abatement potcn tial of the stage. and wastewater treatment . The apparent departure of observed behavior from theo rctical predictions is ascribable to the fact that.Oxygen-stagc solids find their way back into the oxygen stage because it is pan ofaclosedcountercurreni washingsystem This is necessary lo capitalize on the envi ronmcntal benefits obtainable by prevent 5 1Pu| . the digester and the oxygen stage. In the absence of an excess of alkali. 5. 4. Oxygen-stagc washer losses enter the chlorination stage and become unrecoverable The effects ofpre. even the design of other equipment is affected in significant ways that must be considered before the change is made. Filtrate from post-oxygen washing. not the ratio of theirdry weight to the dry pulp weight. partially displaces black liquor from the pulp before it enters the oxygen stage.The desirability of effi-dent pie-oxygen washing is determined by the effect of high solids carryover into the oxygen stage. At each level of black liquor addition. have a detrimental effect. In each experi ment.mi. which suggests that viscosity loss is incurred when the COD concentration is higher than about 20 g/L.ing discharge of oxygen stage solids lo the environmeiK.s conducted at both nsistencies. which is therefore of interest.directly or by way of the con veniional pari of (he bleach pi. Figure 20 shows the results of determinations of their chemical oxygen demand (COO). a surro.perimcni.gate for total organic dissolved solids concentration. Greenwood (64) has reported data ob taincd by the laboratory bleaching of soft wood kraft pulp at 25% consistency in (he presence of systematically varied amounts of dissolved solids (Fig. To est this possibility. at different levels of black liquor carryover. Experiments of the same type at 10% consistency provided the data shown in Hg. wiih and without recirculation. despite scatter in the data. Apparently. a fresh sample of pulp was diluted with filtrate from (he previous experiment simulating oxygen stage solids recirculation and fresh black liquor was added. as would occur in praclice. The higher tolerance of the medium. I Uglily efficient post-oxygen washing is desirable from the environmental stand point but results in maximum recycle of oxygen-stage solids. simulating carryover from the brown stock washers. combinations of recircu lated solids with up to 40 kg/t black liquor solids were tolerated without loss of selec. plotted against the viscosity loss in excess of that observed when fully washed pulp was bleached. combinations of the two types of dissolved solids. 19. On the other hand.consistency system for dissolved solids sug. but higher levels had a detrimental effect. the oxygen-stage was repeated four times.A definite relationship is apparent. carryover of as much as 60 kg/< black liquor solids has no effect on selectivity. 18).gests that the important parameter is their concentration in the liquor. in the absence of recirculation of oxygen-stage solids.tivity. liquors were removed from the pulps after the oxygen stage in ex. . In this case. used to wash (he pulp. If white liquor were used to satisfy. and an addiiional three washing Mages after. which are known to improve pulp cleanliness. 5-2 Screening Placing the oxygen stage in the brown stock washing area makes it necessary to close the screen room water circuit to avoid the loss of oxygen-stage solids. Leaving the knots in the pulp during medium-consistency oxygen bleaching creates the risk of break ing them up in the mixer thereby making them more difficult to remove later. Most mills screen the pulp before oxygen bleaching. together with the chlorine dioxide generator spent acid. represented by perfectly efficient brown stock washing. The screen room may be located before orafterthc oxygen stage.Greenwood (64) bus performed calculations to demonstrate the practical implication of these phenomena. For this reason. there does not appear to be a major effect of screen room location on pulp cleanliness. 78. brightness stability. systems is to employ two washers (or in-digester washing and one washer) before the press (54). This emphasizes the importance of good washing before oxygen bleaching. washing capacity can be added to allow a corresponding decrease in the dilution factor. washer to replace the press. and refining energy requirements arc at least neutral and in some cases slightly beneficial. In the ideal case. The corresponding figure for medium-consistency systems is higher. In medium-consistency systerns these are augmented by an additional . an increase of about 5% in the capacity of the causticizing and lime reburning systems would be needed. heat balance. the dissolved solids entering the oxygen stage consist only of recirculated oxygen stage solids.3 Implications for the recovery system Recycling of organic material dissolved In the oxygen stage to the recovery furnace typically increases the load on the furnace by 3% for softwoods and 2% for hardwoods. because of the higher steam usage in medium-consistency reactors. 79). would probably exceed the requirement for sodium makeup to the liquor system. pitch. Because screen room deckers typically do not operate well above 80°C locating the screen room before the oxy gt*n stage necessitates cooling of the oxy gcn stage filtrate before using it on the screen room washer. requires an increase in the efficiency of the brown stock washing system before the oxygen stage to keep the COD levels in the oxygen stage below that at which viscosity is affected.Two washers arc often employed after the oxygen stage. essential for environmental reasons. increasing overall steam consumption (64). and because it is usually less expensive than purchased caustic. namely that highly efficient washing after the oxygen stage. 6. Greenwood's calculations demonstrate the desirability of three washing stages before..Although the viscosity of the oxygen bleached pulps may be lower (as much as 4 mPa-s on the ТЛРР1 T230 scale). The rcsult ing potential for foaming problems normally dictates the use of pressure screens. their tear factor at a given tensile strength is generally the same as that of conventionally bleached pulps.the total alkali requirements of the oxygen stage. . Other wise. Alternatively. the amount added.The increase in steam generaiion is somewhat less because of the lower heating value of oxidized solids (54). 5. If pure sodium hydroxide were used as alkali in the oxygen stage. Refining energy requirements have been observed to decrease upon installation of an oxygen stage (79). The additional water added with the alkali and as direct steam in the oxygen stage leads to additional evaporator steam requirements of about 4% when high-consistency systems are used. oxidized white liquor is usually used as a partial or sole alkali source. Fmpor tant factors to consider in locating the screen room are effects on cleanliness.but some systems have three. Pulp quality The strength equivalence of conventionally bleached pulps and pulps that have been oxygen delignificd to an extent of about 50% before conventional bleaching is well-documented (66. A further increase of about 1 % results from improved recover)' of black liquor solids which can be credited to the additional washing stages. and capital costs. but there are many examples of the screen room be ing located after the oxygen stage.The effect on cleanliness may be indirect inasmuch as oxygen bleaching is usually followed by short bleaching sequences consisting of high concentration chlorine dioxide stages. perhaps as much as 10%. The effects of oxygen bleaching on pulp cleanliness. the oxygen stage. 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