THE FATE OF CHLORINE SPECIES DURING HIGH TEMPERATURE CHLORINE DIOXIDE BLEACHING

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1 THE FATE OF CHLORINE SPECIES DURING HIGH TEMPERATURE CHLORINE DIOXIDE BLEACHING Gustavo Ventorim, Jorge L. Colodette, Kátia M. M. Eiras, Universidade Federal de Viçosa,Brazil Key words: Chlorine dioxide, bleaching, high temperature, formaldehyde, chlorate, chlorite, chloride, AOX, OX Summary Losses of chlorine dioxide do occur during ECF bleaching by side reactions that produce chlorite, chlorate and other inactive chlorine species. The fate of chlorine species under "normal" chlorine dioxide bleaching is quite well known but not so under high temperature (>90ºC). This study evaluated the fate of chlorine species and bleaching/delignification performances of "normal" and high temperature chlorine dioxide stages for eucalypt kraft-o 2 pulps. The normal D (30min/60 C) and D HT (120min/95 C) bleaching stages were compared under similar conditions, after a mild pulp alkali extraction (E). At end ph 3, the DE treated pulp showed 2.5% ISO higher brightness and 46% higher kappa number (1.9 units) as compared to D HT E treated pulp. Highest DE and D HT E bleaching performances are achieved at end ph 4.0 and 3.0, respectively. For a similar ClO 2 dose (kappa factor 0.20 at end ph 3.0) D HT E bleaching produced 46.3% and 9% less AOX and OX in the filtrate and pulp, respectively, in relation to the DE. The high temperature chlorine dioxide treatment generates less chlorinated organics and chlorate and more chloride ions. No chlorite is formed at end ph 3.0 both in DE and D HT E treatments. Increasing D-stage ph from 3 to 5 reduces formation of chlorinated organics (OX and AOX) and chlorate but increases formation of chlorite. Alkaline extraction (E) reduces pulp organically bound chlorine (OX) by about 59% after the D stage and by about 44% after the D HT stage. Formation of AOX in extraction stage filtrates is about 17 and 22% of that in the D and D HT stage filtrates, respectively. Chloride formation in the extraction stage is about 17% of that in the D stage and about 10% of that in the D HT stage. Formation of chlorate in the extraction filtrates was only 8% of that formed in the D or D HT stages. Addition of 1% formaldehyde to chorine dioxide bleaching improves both DE and D HT E performances, but more so in the former case, particularly at end ph 4. Address of the authors;, Gustavo Ventorim, Doctoral Student, Jorge L. Colodette, Full Professor, Kátia M. M. Eiras, Doctoral Student, Universidade Federal de Viçosa, Viçosa MG, Brazil. Ventorim@ufv.br, Colodett@ufv.br, Katia.eiras@buynet.com.br INTRODUCTION Elemental chlorine free (ECF) bleaching of chemical pulps is currently the dominant technology in the high brightness market pulp segment. About 75% of the world s bleached chemical pulp is produced by this technology, with the proportion expected to grow even more, given the trend toward conversion to ECF in Japan and Brazil. ECF pulp already accounts for over 70% of the Brazilian production. ECF processes have proven efficient, especially for bleaching softwood pulps. ECF bleaching is also efficient for oxygen delignified eucalypt kraft pulp, but there is still room for improvement. The fundamental difference between softwood and eucalypt fibers is in the basis for the kappa number. The basis for the softwood pulp kappa is mainly lignin, while modern mill eucalypt kraft-o 2 pulp kappa is largely caused by the hexenuronic acids (Costa, Colodette 2002). Hot acid hydrolysis - A HT (Vuorinen et al. 1996) and hot chlorine dioxide bleaching (D HT ) (Lachenal, Chirat 2000) technologies, especially suited for removing hexenuronic acids (HexA s), have contributed significantly to improving ECF bleaching of eucalypt fibers. However, ECF bleaching of eucalypt kraft-o 2 pulps can still be improved. For example, a fraction of the chlorine dioxide used in bleaching is converted to chlorite and chlorate. These two byproducts from chlorine dioxide are ineffective in bleaching the pulp. Addition of sodium chloride to the system can suppress formation of chlorite (Rapson, Anderson 1978 cited by Jiang et al. 2002), but this approach 1

2 is sensitive to pulp origin and requires a concentration of chloride as high as 5000 ppm (Reeve, Weishar 1992 cited by Jiang et al. 2002), which can be technologically unacceptable. The reconversion of chlorite into chlorine dioxide through use of reducers in the bleaching process appears to be a viable option (Jiang et al. 2002), but the best alternative is, usually, to operate at the optimum ph for chlorine dioxide use in order to minimize chlorite and chlorate formation (Reeve 1992 cited by Jiang et al ). The chorine dioxide speciation into chlorite, chlorate, chloride and organically bound chlorine, etc. is quite well investigated for "normal (60-70 C)" chlorine dioxide delignification/bleaching and brightening (Chang et al. 2001; Ni 1992; Rapson, Strumila 1979; Strumila, Rapson 1976). However, this information is not yet available for hot chlorine dioxide delignification/bleaching (>90 C). Hence, the objective of the present work was to compare chlorine dioxide performance and speciation during bleaching at normal (D, 60 C/30min) and high time/temperature (D HT, 95 C/120min) conditions. Also investigated was the addition of formaldehyde into chlorine dioxide bleaching at normal and high temperature conditions. MATERIAL AND METHODS Industrial oxygen delignified eucalypt kraft pulp samples (kraft-o 2 pulp) were used. The characteristics of the different kraft-o 2 pulp samples are presented in the footnotes of the respective result tables. The chorine dioxide stage (D or D HT ) was carried out, in the presence or absence of formaldehyde, with 300 g oven dry samples in a model Mark V mixer/reactor (Quantum Technologies Inc.). In these stages formaldehyde and/or chlorine dioxide was added to the pulp along with appropriate amounts of NaOH or H 2 SO 4 in order to produce the desired ph value at the end of the treatment. The required doses of acid or base were determined in previous experiments using the trial and error technique. Conventional alkaline extraction was run with 280 g pulp samples in the same reactor. After each bleaching stage, run in duplicate, the samples were washed with excess distilled water. Reagent doses are expressed in percent, based on oven dried pulp weight. Pulp kappa number, viscosity, brightness and brightness stability values were measured according to Tappi procedures. Chloride, chlorite and chlorate were analyzed by ion chromatography (IC model LC- 10AD VP Shimadzu). Pulp OX and filtrate AOX values were measured in an absorbable organic halogen analyzer (ECS 1600 Euroglas), according to SCAN procedures. Filtrate total organic carbon (TOC) values were measured directly in a Shimadizu model 5000A TOC analyzer. Overall bleaching yield was determined indirectly by analyzing TOC in the bleaching filtrates and converting carbon loss into yield loss through calibration equations. RESULTS AND DISCUSSION High Temperature Chorine Dioxide Bleaching Performance The first chlorine dioxide bleaching stage (D) is usually carried out at a temperature of C for min (Reeve 1996). More recently, hot chlorine dioxide bleaching (D HT ) has been proposed at temperature of C for min (Lachenal, Chirat 2000; Ragnar, Törngren 2002; Ragnar, Dahlhöf 2002; Ragnar 2003; Ragnar, Lindström 2004; Eiras, Colodette 2003; Ragnar 2004). The effect of reaction time on the performance and speciation of chlorine dioxide during bleaching at 60 and 95 C and ph 3.0 are shown in Figures 1 to 5. Average results for the fixed conditions of 30min/60 C (D) and 120min/95 C (D HT ) are presented in Table 1. Notice that filtrate analyses were carried out for each individual stage. For the fixed conditions at ph 3 (Table 1), the first chlorine dioxide stage at high temperature (D HT ) decreased the brightness by 2.5% ISO and kappa number by 46% (1.9 units) measured after mild extraction (E) as compared to the conventional D stage, indicating that the D HT E treatment is more efficient than the DE one in reducing the pulp kappa number and less efficient in pulp brightening, as previously demonstrated (Eiras, Colodette 2003; Ragnar, Lindström 2004). The higher kappa number drop in the D HT E treatment as compared to the DE has been explained by the greater removal of HexA s in the first case under the conditions of higher temperature and longer reaction time (Lachenal, Chirat 2000). This trend is clear in Figure 1, in which a continuous kappa number reduction is observed after 30 min reaction at 95 C. The lower brightness obtained after D HT E (Fig. 2) is explained by the brightness reversion reactions caused by maintaining the pulp at 2

3 high temperature/time in the complete absence of chlorine dioxide (Eiras, Colodette 2003); the hot acid treatment induces formation of new lignin phenolic hydroxyl groups, which may give rise to new chromophores (Uchida et al. 1999). On the other hand, the D HT E treatment resulted in 11.4% lower post extraction pulp viscosity as compared to the DE one; note that viscosity continues decreasing with time after 30 min of reaction at 95 C (Fig. 3). Similar viscosity losses have been reported for different wood species by Ragnar Pulp exposed to high time/temperature reaction and acid ph may undergo slight carbohydrate hydrolysis. The significant viscosity penalties shown are more likely caused by the actions of chorine and hypochlorous acid derived from chlorine dioxide, which are magnified at the high temperature. A higher yield loss occurred across the D HT E treatment in relation to the DE one (Table 1). This trend has been observed in another study (Eiras, Colodette 2003) and attributed to the lower kappa number of the pulp treated at the higher time/temperature condition. Yield loss is directly related to the amount of organic carbon present in bleaching filtrates, which can be measured by the Total Organic Carbon (TOC) technique. Each ~4 kg C/t of TOC represents about 1% yield loss (our experience). In Figure 4 the TOC values continue to rise after 30 min reaction at a temperature of 95 ºC indicating additional dissolution or organic matter. It has been determined that one kappa number unit of softwood pulp is equivalent to roughly 0.15% of lignin. However, such relationship is not very appropriate to estimate yield loss in this study since the increase in TOC values across the D HT E treatment derives in large part from HexA s hydrolyzed under the high time/temperature condition. Vuorinen et al reported that one kappa unit is equivalent to approximately 10 mmol/kg pulp or 0.16% of HexA s. Hence, it is possible to make a good approximation on the theoretical yield losses due to the additional kappa number reduction in the D HT E treatment. The kappa number of the DE and D HT E treated pulps were 4.1. and 2.2, respectively (Table 1), with the 1.9 kappa units difference being equivalent to 0.29% lower yield for the D HT E treated pulp if the above relationship holds. In practice, the yield difference was 0.57% (Table 1). Thus, the D HT E stage removed 0.28% more material than the theoretical in order to drop 1.9 kappa units. This loss may be compensated in the end since the D HT E treated pulp will require less chemicals downstream in the bleaching process in relation to the DE treated one. However, precise yield loss quantification is possible only after full pulp bleaching, which was not the case here. 6 Extraction Kappa No DE D HT E Reaction Time, min Fig 1: Semi log plot of pulp kappa number values after DE and D HT E bleaching versus reaction time (ClO 2 treatment at end ph 3.0 and kappa factor 0.20). 3

4 74 Brightness, % ISO D HT E DE Reaction Time, min Fig 2: Semi log plot of pulp brightness values after DE and D HT E bleaching versus reaction time (ClO 2 treatment at end ph 3.0 and kappa factor 0.20). Pulp Viscosity, dm 3 /kg Reaction time, min D HT E DE Fig. 3: Semi log plot of pulp viscosity values after DE and D HT E bleaching versus reaction time (ClO 2 treatment at end ph 3.0 and kappa factor 0.20). 4

5 Total Organic Carbon, kg C/ t pulp DE D HT E Reaction Time, min Fig 4: Semi log plot of DE and D HT E combined filtrate TOC values versus reaction time (ClO 2 treatment at end ph 3.0 and kappa factor 0.20). Chlorine Dioxide Speciation at High Temperature Bleaching Figure 5 illustrates the effect of reaction time on chlorine dioxide speciation and fate. It can be seen that the quantity of AOX present in the D HT E filtrate is about 50% lower than that found in the DE filtrate (Table 1). The reduction in the AOX values caused by increasing reaction time/temperature is apparently due to two factors: 1) slightly lower chlorinated organics generation and (2) conversion of organically bound chlorine to chloride. The slightly lower generation of chlorinated organics under conditions of higher time/temperature is evident in Figure 5 and is explained by the partial acid hydrolysis of HexA s, which are important sources of AOX (Costa, Colodette 2002; Freire et al. 2004). Evidence for the second hypothesis, the most significant one, is the reduction of AOX levels and simultaneous increase in filtrate chloride levels, with the increase in reaction time at 95 C (Fig. 5). At the end of the reaction, a 22.5% higher level of chloride was found in D HT E filtrates (Table 1). This additional chloride probably arose from degradation of chlorinated organic compounds under the condition of high time/temperature. The conversion of chlorine dioxide into chlorate was not significantly influenced by reaction time/temperature and was only 7% lower in the D HT E process, while chlorite was not detected in either case. 5

6 g Cl - /t pulp Reaction Time, min D HT E Chloride DE Chloride DE Chlorate D HT E Chlorate D HT E AOX DE AOX Figure 5: Semi log plot of DE and D HT E combined filtrate chlorine species values versus reaction time (ClO 2 treatment at end ph 3.0 and kappa factor 0.20). The amount of OX bound to the pulp after the extraction stage was slightly influenced by the high reaction time/temperature condition. The pulp OX value after D HT E was only 9% lower than after DE (Table 1). It should be noted that the pulp OX content was 34% lower after D HT than after D. Apparently, a significant fraction of the pulp OX generated in the chlorine dioxide stage is destroyed in the alkaline extraction stage. It has been shown that pulp OX content is directly related to the residual lignin content (Costa, Colodette 2002). Therefore, it is reasonable to assume that the lignin content in the pulps after DE and D HT E did not differ significantly. The difference between pulp kappa number values in the two cases was probably due to differences in their HexA s contents. It is interesting to note in Table 1 that of the total chlorine dioxide applied in the D or D HT stages (7.6 kg/t of pulp), which is equivalent to g Cl - /t of pulp, 92% and 85% were recovered in the post extraction pulp and combined filtrates, for the D HT E and DE treatments, respectively. At present we have no explanation for the higher level of recovery in the D HT E process. The fraction not recovered was probably lost in the form of volatile compounds such as elemental chlorine, chloroform, etc., and partly as measurement errors; it is not unlikely that at the higher temperature less volatile compounds are formed given the fast reaction of chlorine dioxide with lignin and HexA s. The greatest fraction of chlorine dioxide was converted to chloride, regardless of the temperature. Chlorate represented the second largest part and the organically bound chlorine, measured as AOX and OX, the least significant fraction. Effect of alkali extraction It can also be seen in Table 1 that, regardless of ClO 2 stage time and temperature, the alkaline extraction step reduces the kappa number (~0.6 units) and increases pulp brightness (1,5-4,5% ISO). Brightness gain due to extraction is most significant after the D HT stage (4.5% ISO). Extraction slightly reduces the pulp viscosity (20 dm 3 /kg) and results in about 0.9% yield loss. Alkali extraction reduces pulp organically bound chlorine by about 60% after the D stage and by about 45% after the D HT stage. Formation of AOX in extraction stage filtrates is about 17% and 22% of that in the D or D HT stage filtrates, respectively. Chloride formation is about 17% of that in the D stage and about 10% of that in the D HT stage. These results suggest that recycling extraction stage filtrate is less difficult after D HT than after D stage. Formation of chlorates in the extraction filtrates was only 8% of that formed in the D or D HT stages. 6

7 Table 1. Effect of time/temperature on chlorine dioxide performance and fate in bleaching of kraft-o 2 pulp* Results DE Treatment D HT E Treatment D E D HT E ClO 2 applied, kg/t 7.6 (3997 g (3997 g - Cl - /t) Cl - /t) Temperature, ºC Time, min Kappa Number Brightness, % ISO Viscosity, dm 3 /kg Yield loss, % Pulp OX, g Cl - /t 281** ** 104 Filtrate TOC, kg C/t Filtrate AOX, g Cl - /t Filtrate chloride, g Cl - /t Filtrate chlorate, g Cl - /t Filtrate chlorite, g Cl - /t not detected not detected not detected not detected Total chlorine compounds, g Cl - /t *Kraft-O 2 pulp: kappa number 10, brightness 49.8 % ISO, viscosity 1074 dm 3 /kg; ** Not included in total chlorine compounds calculation; D and D HT : 10% cst, end ph 3.0, kappa factor 0.20; E: 10% cst, ph , 12 kg/t NaOH. Effect of ph Increasing D or D HT stage end ph from 3 to 5 significantly reduces chlorine dioxide effect on kappa number (Table 2). The negative impact is more pronounced in the D HT stage because of the lower HexA s hydrolysis rate at ph 5. However, operation of the first chlorine dioxide stage at ph 5 results in higher pulp brightnesses and viscosities. Operation at ph 5 slightly reduces formation of pulp OX but reduce AOX significantly (~36%) both for DE and D HT E treatments. In addition, the operation at the higher ph value decreases formation of chlorate (22% for DE and 17% for D HT E) but increases formation of chlorides (13% for DE and 8% for D HT E) and promotes chlorite generation. The formation of chlorite (220 g/t for DE and 210 g/t for D HT E) has a negative impact on kappa number since chorine dioxide in this form is not effective to remove lignin or HexA s, although it has a pulp brightening effect, which is seen by the higher pulp brightness at end ph 5.0. Table 2. Effect of D and D HT stage ph on chlorine dioxide performance and fate in bleaching of eucalypt kraft-o 2 pulp* Results After DE treatment After D HT E treatment D or D HT stage end ph Kappa Number Brightness, % ISO Viscosity, dm 3 /kg Pulp OX, g Cl - /t Filtrate AOX, g Cl - /t Filtrate chloride, g Cl - /t Filtrate chlorate, g Cl - /t Filtrate chlorite, g Cl - /t not detected 220 not detected 210 Total chlorine compounds, g Cl - /t *Kraft-O 2 pulp: kappa number 10, brightness 49.8 % ISO, viscosity 1074 dm 3 /kg; D: 10% cst, 60 ºC, 30 min, kappa factor 0.20; D HT : 10% cst, 95 ºC, 120 min, kappa factor 0.20; E: 10% cst, end ph , 12 kg/t NaOH. The effect of ph is also shown in Table 3 for a different kraft-o 2 eucalypt pulp sample. D stage (30 min/60 C) efficiency, measured by post extraction pulp kappa number is highest at end ph 4, in the presence or absence of formaldehyde. The positive effect of increasing ph from 3 to 4 is explained by minimizing conversion of chlorine dioxide to chlorate, and consequently, loss of this reagent. However, 7

8 the increase from ph 4 to 5 reduces efficiency of this stage through favoring formation of chlorite that is not effective in reducing pulp kappa number. An optimum end ph for bleaching hardwood fiber of about 4 has been reported by Chandranupap and Nguyen, D HT stage efficiency, as measured by post D HT E kappa number, is highest at end ph 3 because of the greater efficiency of HexA s removal under the low ph, high temperature/time conditions. This indicates that the effect of dioxide speciation is overridden by the effect of acid hydrolysis of HexA s. Therefore, one should consider running the D HT stage in two steps: (1) a first step without chlorine dioxide at ph 3 to remove the HexA s and (2) a second step with dioxide at ph 4 to take maximum advantage of the active form of this reagent. This concept is easily applied using the A HT /D technology proposed and patented by Henricson, However, in industrial practice for eucalypt kraft O 2 pulp, D HT and A HT /D technologies tend to produce similar results, even when the ph control aspects are taken into account. This no difference may be explained assuming that lignin becomes less reactive towards chlorine dioxide when treated by a hot acid stage (A HT ) such as in the A HT /D technology and this overshadows the potential benefits gained by running the hot acid (A HT ) and chlorine dioxide stages separately and at different ph values. A recent study (Ragnar, Lindström 2004) showed that the D HT technology can actually be more efficient than the A HT /D one, a matter of much debate. Impact of Formaldehyde on High Temperature Chlorine Dioxide Bleaching It has been suggested that using formaldehyde as additive in the chorine dioxide stage may convert some byproducts formed in side reactions back to chlorine dioxide, thereby increasing the efficiency of this reagent (Jiang et al. 2002). Table 3 presents results of pulp pre-bleaching using the DE and D HT E treatments, with the D stage carried out at end ph values of 3, 4 and 5, with and without addition of 1% formaldehyde based on pulp dry weight. Addition of 1% formaldehyde in the chlorine dioxide stage run at end ph 3 to 5 reduces the post DE or D HT E kappa number (Table 3). However, the benefit from formaldehyde addition is most significant at ph 4 in the DE process. The reason why formaldehyde is less effective when the dioxide stage is performed at high time/temperature (D HT ) is not clear. The positive effect of formaldehyde has been attributed (Jiang et al. 2002) to its capacity to regenerate chlorine dioxide from chlorite, according to the reaction [1]. Likely, less chlorite is formed in the D HT treatment in relation to the D one since chlorine dioxide is consumed rather quickly in the former case. Note that the products of reaction [1] are acidic in nature and will drop the reaction ph if sufficient NaOH is not previously added to the system. In this study, the results were compared at equal end ph values so as to avoid such potential problem. HCHO + 3HClO 2 2 ClO 2 + HCO 2 H + HCl + H 2 O [1] It should be noted that post DE or D HT E brightnesses are slightly higher in the treatments without formaldehyde but this additive does not influence pulp viscosity. Use of formaldehyde in the dioxide stage appears to slightly reduce pulp OX formation, regardless of the ph and time/temperature of reaction. Table 3. Results of DE and D HT E bleaching of an eucalypt kraft-o 2 pulp* with varying end ph in the presence and absence of 1% formaldehyde (on pulp dry weight) in ClO 2 treatment Results After DE After D HT E No Formaldehyde 1% Formaldehyde No Formaldehyde 1% Formaldehyde D stage end ph Kappa Number Brightness, % ISO Viscosity, dm 3 /kg Pulp OX, g Cl - /t *Kraft-O 2 pulp: kappa number 11.2, brightness 48.6 % ISO, viscosity 1194 dm 3 /kg; D: 10% cst; 60 o C; 30min; 9.4 kg/t ClO 2 ; 2 kg/t H 2 SO 4 (ph 3); 6 kg/t H 2 SO 4 (ph 4); 10 kg/t H 2 SO 4 (ph 5); D HT : 10% cst, 95 o C, 120min; 9.4 kg/t ClO 2 ; 2 kg/t H 2 SO 4 (ph 3); 6 kg/t H 2 SO 4 (ph 4); 10 kg/t H 2 SO 4 (ph 5); E: 10% cst; 70 o C; 60min; 10kg/t NaOH; final ph

9 CONCLUSIONS The results of this study permit drawing the following main conclusions: At ph 3, a DE treated eucalypt kraft-o 2 pulp showed 2.5% ISO higher brightness and 46% higher kappa number (1.9 units) as compared to D HT E treated pulp. "Normal" D stage reaction is more effective at ph 4.0 whereas D HT stage reaction is more effective at ph 3.0. At ph 3.0 the D HT stage (120 min/95 C) produces about 40 to 50% less AOX than the D stage (30 min/60 C) at constant ClO 2 charge (kappa factor 0.20). The higher reaction time/temperature condition in the D-stage not only generates less chlorinated organics but also decompose these compounds, giving rise to chlorides. Addition of 10 kg/t formaldehyde to a conventional (30min/60 C) chlorine dioxide stage at ph 4 results in a 1.9% ISO lower brightness and a 26% lower kappa (1.2 units) after extraction. When added to a hot dioxide stage (120 min/95 C) the benefit is less significant, in the order of 17% lower kappa (0.7 kappa units) after extraction. LITERATURE Chandranupap, P. And Nguyen, K. (2000): Effect of ph on kinetics and bleaching efficiency of chlorine dioxide delignification. Appita J. 53 (2), 108. Chang, H-m, Svensen, D.R., Jameel, H. and Kadla, J.F. (2001): recent advances in chemistry of chlorine dioxide. In: "7 th Brazilian Symposium on the Chemistry of Lignins and Other Wood Components Proceedings". Oral Sessions. Belo Horizonte, Brazil. UFV Press, Viçosa, pp Costa, M.M., Colodette, J.L. (2002): The effect of kraft pulp composition on its bleach ability. In: " International Pulp Bleaching Conference Proceedings". Oral Sessions. Portland, OR. Tappi Press, Atlanta, pp Eiras, K.M.M., Colodette, J.L. (2003): Eucalypt kraft pulp bleaching with chlorine dioxide at high temperature, JPPS 29(2), 64. Freire, C.S.R., Silvestre, A..J.D., Neto, C.P., Cavaleiro, J.A.S. (2004): Glucuronoxylan-derived chlorinated compounds in filtrates from chlorine dioxide bleaching: a comparative study between eucalypt (E.globulus) and birch (Betula spp.) kraft pulps. Appita J. 57(1), 40. Henricson, K. (1997): AHL STAGE A new bleaching stage for kappa reduction and metal profile control. In: "International Emerging Technologies Conference and Exhibition". Orlando, FL, Miller Freeman Press, San Francisco. Jiang, Z., Lierop, B. and Berry, R. (2002): improving chlorine dioxide bleaching with aldehydes. In: "2002 International Pulp Bleaching Conference Proceedings". Oral Sessions. Portland, OR. Tappi Press, Atlanta, pp Lachenal, D. & Chirat, C. (2000): High temperature chorine dioxide bleaching of hardwood kraft pulp. Tappi J. 83 (8), 96. Ni, Y. (1992): A Fundamental Study of Chlorine Dioxide Bleaching of Kraft Pulps, Ph.D. Dissertation, McGill Univ., Montreal. Ragnar, M. and Törngren, A. (2002): Ways to reduce the amount or organically bound chlorine in the bleached pulp and the AOX discharges from ECF bleaching. Nordic Pulp and paper Res. J. 17(3), 234. Ragnar, M. and Dahllöf, H. (2002): ECF bleaching of Eucalypt kraft pulp - bleaching chemical needs and yellowing characteristics of different sequences. Nordic Pulp and paper Res. J. 17(3), 228. Ragnar, M. (2003): A comparative study of hot versus conventional chlorine dioxide bleaching for different wood species. Appita J. 56(6), 471. Ragnar, M. and Lindstrom, M.E. (2004): A comparison of emerging technologies: hot chlorine dioxide bleaching versus hot acid treatment. Paperi Ja Puu - Paper and Timber 86(1), 39. Ragnar, M. (2004): On the theoretical basis for the low bleaching chemical requirement of hot chlorine dioxide bleaching of hardwood kraft pulp. Nordic Pulp and Paper Res. J. 19(1), 78. Rapson, W.H., Strumila, G.B. (1979): In: "The Bleaching of Pulp", Edited R.P. Singh, third edition, Tappi Press, Atlanta, pp

10 Reeve, D.W. (1996): Section IV: The Technology of Chemical Pulp Bleaching. Chapter 8: Chlorine Dioxide in Bleaching Stages. In " Pulp Bleaching Principles and Practice" Edited C.W. Dence and D.W. Reeve. Tappi Press, Atlanta, pp Strumila, G.B. and Raspon, W.H. (1976): Chlorine Dioxide Oxidation of lignin Model Phenols. In: "Canadian Wood Chem. Symposium". CPPA Press, Montreal. pp Uchida, Y., Miura, T., Iwasaki, M. (1999): Acid treatment under pressurized oxygen gas. In:"Tappi Pulping Conference Proceedings". Orlando, FL. Tappi Press, Atlanta, pp Vuorinen, T., Buchert, J., Teleman, A., Tenkanen, M., Fagerstrom, P. (1996): Selective hydrolysis of hexenuronic acid groups and its application in ECF and TCF bleaching of kraft pulps. In: International Pulp Bleaching Conference Proceedings". Oral Sessions. Washington, DC. Tappi Press, Atlanta, pp