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Effect of Pulping Conditions on the ECF Bleachability of Eucalyptus globulus Kraft Pulps Carlos Pascoal Neto,*,† Dmitry V. Evtuguin,† Fernanda P. Furtado,‡ and Anto´ nio P. Mendes Sousa‡ Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal, and Raiz, Quinta de S. Francisco, 3800 Eixo, Aveiro, Portugal
The effect of Eucalyptus globulus wood kraft pulping conditions on the chlorine dioxide consumption in elemental chlorine free bleaching of pulps was investigated. When the pulping is extended (kappa number decreases), the bleachability [as (OXE/ton)/kappa number] is reduced, suggesting that residual lignin and other oxidizable unsaturated structures in pulp become less reactive as their content in the pulp decreases. For the same unbleached pulp kappa number, the increase in active alkali and sulfidity used in pulping improves bleachability, whereas the pulping temperature (150-170 °C) does not affect the bleaching response. The increase in liquorto-wood ratio has a detrimental effect on the bleachability. No clear correlation could be established between unbleached pulp brightness and its bleaching response. Overall, the results obtained show a good correlation between the integral of the relative reaction rate constant (H factor) used in the pulping and pulp bleachability, with a low H factor being beneficial to the bleaching response of the pulp. Introduction The industrial processes used to convert wood into bleached cellulosic fibers often involves, first, the removal of 92-95% of lignin from wood with a pulping liquor composed essentially of sodium hydroxide and sodium sulfide (kraft pulping process).1 Then, the removal of residual lignin (and other minor pulp components such as hexenuronic acid groups and extractives) and the destruction of chromophores, the bleaching process, can be accomplished by a series of oxidative treatments with chlorine dioxide alternating with alkaline extraction stages (ECF, elemental chlorine free bleaching).2 Alternatively, the bleaching can be carried out without chlorine-based chemicals by using oxidants such as molecular oxygen, hydrogen peroxide, ozone, or peracids (TCF, totally chlorine free bleaching).2 The conditions used in the pulping affect the chemical composition and structure of the unbleached pulps3,4 and, therefore, their bleaching response (consumption of bleaching chemicals to reach a desired brightness). Previous studies on the influence of pulping parameters in the bleaching response of kraft pulps were carried out essentially with softwoods and Scandinavian and North American hardwoods and involved both ECF and TCF bleaching sequences.3-8 In contrast, for Eucalyptus species, the information available on this subject is rather scarce9,10 and not comprehensive. Some apparent conflicts can be observed in the results that are basically attributed to the different type of wood, bleaching processes, and experimental design and conditions used. The DEDED bleaching sequence (where D stands for chlorine dioxide and E for aqueous NaOH extraction stages) is a typical ECF bleaching sequence and the most widely used worldwide.2 Chlorine dioxide is an * To whom correspondence should be addressed. Tel.: +351 234 370693. Fax: +351 234 370084. E-mail:
[email protected]. † University of Aveiro. ‡ Raiz.
expensive chemical, and although much less hazardous than chlorine (Cl2) previously used in bleaching processes, it leads to the formation of minor amounts of organochlorine compounds.2 Thus, the reduction of ClO2 consumption is an important goal of ECF bleached pulp producers. The aim of this work is to investigate how the conditions used in the kraft pulping of Eucalyptus globulus wood affect the bleachability of pulps by a conventional D0E1D1E2D2 bleaching sequence and to identify the pulping conditions maximizing bleachability, pulp yield, and pulp quality. Experimental Section Pulping. Pulping experiments were carried out with industrial-size chips from a 12-year-old E. globulus clone plantation in 5.8-L forced circulation batch digesters equipped with an external electric heating system and temperature control. The reference pulping conditions were as follows: initial temperature of pulping liquor, 40 °C; time to final temperature, 120 min; final temperature, 160 °C; active alkali charge, AA ([(weight of NaOH + Na2S)/weight of wood] × 100), 17% (as Na2O); sulfidity, S ([weight of Na2S/(weight of NaOH + Na2S)] × 100), 28%; liquor-to-wood ratio, L/W [volume of liquor (liters)/weight of wood (kilograms)], 4. In a first series of pulps, only the pulping time was varied, with all other pulping parameters kept constant, to obtain pulps with variable residual lignin contents (expressed by the kappa number). For the remaining series of pulps, pulping parameters/conditions (active alkali charge, sulfidity, temperature, liquor-to-wood ratio, alkali and temperature profile, and pre-extraction of wood chips) were varied separately, with the others kept constant. In these series, the pulping time was adjusted to maintain a constant kappa number of 14 ( 0.3. When variation of the alkali profile was investigated, the total alkaline charge was added in two fractions, the first portion being added in the beginning of the cooking and
10.1021/ie020263x CCC: $22.00 © 2002 American Chemical Society Published on Web 10/26/2002
Ind. Eng. Chem. Res., Vol. 41, No. 24, 2002 6201 Table 1. Conditions and Results of Kraft Cookingsa AAb (% Na2O) 17 17 17 17 17 17 17 17 17 14 21 24 17 17 17 17 17 17 17 17 17
pulping conditions Sc T L/Wd (%) (°C) (L/kg) 28 28 28 28 28 28 28 28 28 28 28 28 15 21 37 28 28 28 28 28 28
160 160 160 160 160 160 160 160 160 160 160 160 160 160 160 150 155 170 160 160 160
4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 3.5 5 8
t (min)
pulp yield (%)
kappa number
viscosity (mL/g)
brightness (%)
ClO2 consumption (%)
145 150 154 160 165 170 180 195 210 190 145 138 185 175 165 255 197 131 170 179 216
56.6 56.9 56.2 55.9 56.3 55.5 55.9 54.5 55.1 56.7 54.8 53.8 55.0 55.3 56.9 56.5 55.4 55.8 52.0 55.7 54.7
17.7 17.0 15.0 14.7 14.8 13.8 12.5 12.5 11.6 13.7 13.8 14.2 13.9 13.7 13.7 13.7 13.8 13.9 14.0 14.3 13.7
1445 1445 1458 1400 1449 1387 1345 1262 1240 1500 1364 1325 1251 1322 1460 1427 1385 1345 1243 1454 1476
45.7 46.6 47.4 47.5 47.6 47.8 47.6 47.6 48.2 45.4 49.5 49.6 48.3 47.4 46.3 46.0 47.3 47.1 46.8 46.7 46.2
4.97 5.10 4.40 4.30 4.30 4.10 4.00 4.00 3.90 4.20 4.01 3.65 4.15 3.90 3.65 3.91 3.83 3.88 4.03 4.29 4.69
a Reference pulping conditions and corresponding results are in italics. b AA ) active alkali. c S ) sulfidity. ratio.
the remaining at the end of the temperature rising period, after removal of the same amount of black liquor so as to keep the liquor-to-wood ratio constant. The temperature profile was modified by preheating the initial pulping liquor and adding it to the reactor at different temperatures. Bleaching. Bleaching experiments (D0E1D1E2D2 sequence) were carried out in closed plastic bags introduced into a water bath with temperature control. Standard industrial bleaching conditions were used. The strategy for the distribution of the ClO2 charge (expressed as “active chlorine”) was as follows: in D0, a predetermined ClO2 charge was used to reach a final kappa number ∼6 (with no residual ClO2 at the end of the stage); the remaining ClO2 charge, predetermined to achieve 90.5% brightness at the end of the bleaching sequence, was distributed between the D1 and D2 stages in proportions of around ∼75 and ∼25%, respectively. ClO2 consumption was expressed as the weight percent of active chlorine in the dry pulp. ClO2 in solution was quantified by conventional iodometric titration. The bleaching conditions were as follows: D0, 25 min, 50 °C, pH ≈ 3.0; E1, 120 min, 70 °C, 2.1% NaOH; D1, 240 min, 70 °C, pH ≈ 3.5; E2, 120 min, 70 °C, 0.6% NaOH; D2, 240 min, 70 °C, pH ≈ 3.5. All stages were carried out at 10% consistency (kilogram of dry pulp per kilogram of suspension). The bleaching response or bleachability, defined as the consumption of oxidant normalized by the amount of residual lignin and other oxidizable structures in the pulp, was expressed as (OXE/ton)/kappa number.11 In this expression, OXE/ton stands for the number of oxidation equivalents (kilograms of active chlorine × 28.20) per ton of unbleached dry pulp, and kappa number (including the contribution of hexenuronic acid) represents the difference between the kappa numbers of the pulp entering the bleaching stage or sequence and the pulp leaving the bleaching stage or sequence). In the calculations of bleachability the kappa number of fully bleached pulps was assumed to be 0. Pulp Characterization. The kappa number (an indication of the content of permanganate oxidizable
d
L/W ) liquor-to-wood
structures in pulp, essentially lignin and, in lower amounts, hexenuronic acid), viscosity (a property related to the degree of polymerization of polysaccharides in pulp), and brightness of the pulps were determined by standard methodologies.12 Results and Discussion Variation of Pulping Extension (Variable Kappa Number). The pulping time was varied while all other pulping parameters were kept constant (reference conditions; see Experimental Section). This allowed for the production of pulps with different extents of delignification (kappa numbers ranging from 17.7 to 11.6). The pulping conditions and results, as well as data on ClO2 consumption in the bleaching process, are shown in Table 1. As expected, the total amount of ClO2 consumed to reach 90.5% brightness in the D0E1D1E2D2 bleaching sequence decreases as the kappa number of the pulps decreases (pulping time increases) (Figure 1). However, when the bleaching response or bleachability, expressed as the number of oxidation equivalents (OXE) consumed per unit reduction in kappa number (oxidant consumption normalized by the content of lignin and other oxidizable structures) is plotted against the kappa number of the unbleached pulp, an increase of this variable (denoting a decrease of bleachability) is observed as the extent of delignification increases (kappa number decreases). Hence, the residual lignin and other oxidizable components remaining in the pulp are progressively more resistant to oxidation as their content in the pulp decreases. This reactivity reduction has been assigned to an enrichment of the “condensed” structure content and a decrease of the β-O-4 structure content of the residual lignins as the pulping/delignification proceeds.13 Variation of Conventional Pulping Parameters (Constant Kappa Number). Four series of pulps were produced by varying separately each one of the following parameters: active alkali (AA), sulfidity (S), temperature, and liquor-to-wood ratio (L/W). All other parameters were kept constant at their reference values. In
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Figure 1. Influence of the kappa number of unbleached pulp on its bleaching response to a D0E1D1E2D2 bleaching sequence (90.5% brightness). (See Table 1 for conditions.)
each case, the pulping time was adjusted to obtain a constant kappa number of 14 ( 0.3. The pulps were then submitted to a standard ECF D0E1D1E2D2 bleaching process. The influence of active alkali in the bleachability of unbleached pulp was investigated in the range of 1424% (Table 1). It was found that the bleachability increases markedly with increasing active alkali (Figure 2). Thus, a high concentration of OH- and HS- anions in the pulping liquor is clearly beneficial to the bleaching response of kraft pulp. At low AA (14%), 4.2% (42.0 kg/ton of pulp) of the ClO2 is consumed to reach 90.5% brightness, compared to 3.65% (36.5 kg/ton of pulp) at 24% AA. This result is in agreement with previous findings showing that a high residual alkali at the end of the cook is beneficial to bleachability.10 However, the improvement of bleachability with AA is accompanied by significant decreases in the yield (5%) and viscosity (12%) of the unbleached pulp (Table 1), which is reflected in the yield and mechanical properties of the bleached pulp (results not shown). Regarding sulfidity, a series of kraft pulps was produced with S ranging between 15 and 37% (Table 1). The amount of OXE consumed per ton of pulp to reach 90.5% brightness decreases with increasing sulfidity (Figure 2), showing that, at constant active alkali, the bleachability is improved by an increase in the HS-/ OH- concentration ratio. The use of high sulfidity (37%) allows for savings of 5.0 kg of ClO2 per ton of pulp to achieve 90.5% brightness, 12% less than with low sulfidity charge (15%) (Table 1). Also, the increase of S leads, as expected, to significant improvements in the unbleached pulp yield (3.5%) and pulp viscosity (17%). The use of high sulfidity (up to the highest level investigated, 37%) is, thus, beneficial from the point of view of the production yield, pulp quality, and bleaching chemical savings. A series of four pulps was produced at various pulping temperatures in the range 150-170 °C (Table 1). When
Figure 2. Influence of kraft pulping parameters on the bleaching response of pulps with kappa number 14 ( 0.3 to a D0E1D1E2D2 bleaching sequence (90.5% brightness). (See Table 1 for conditions.)
these pulps were submitted to the bleaching sequence, no significant differences were observed in the corresponding bleaching responses (Figure 2). However, the temperature increase had a slight negative effect on the pulp yield and pulp viscosity (Table 1). The influence of the liquor-to-wood ratio on the bleaching response of unbleached pulps was investigated in the range 3.5-8.0, with all other pulping parameters kept constant (Table 1). The bleachability of the pulp drops as the relative proportion of liquor to wood in the pulping reactor increases (Figure 2). Such a result is in agreement with the previously observed behavior of the bleaching response with the active alkali
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Figure 3. Influence of active alkali concentration (corresponding to different AA or L/W values) on the bleaching response of pulps with kappa number 14 ( 0.3 to a D0E1D1E2D2 bleaching sequence (90.5% brightness). (See Table 1 for conditions.)
Figure 4. Bleaching response in different D stages of the D0E1D1E2D2 bleaching sequence (90.5% brightness) as a function of sulfidity used to produce the unbleached kraft pulp.
content (with constant L/W). When L/W is increased at constant AA, the concentrations of OH- and HS- anions are lowered, leading to a lower bleaching response of pulps. It might be expected that, in the presence of higher volumes of liquor, the lower concentration of lignin in the black liquor would be beneficial to bleachability, decreasing the extent of lignin side reactions in solution (including condensation) and its redeposition in fibers. However, for the pulping conditions used, this was not observed, certainly because the effect of the lower concentration of reagents is dominant. In fact, when results shown in Figure 2 for AA and L/W are summarized in a single graph using concentration in grams of Na2O per liter of liquor (Figure 3), it becomes clear that the alkali concentration is the variable determining the bleaching response. The higher alkali concentration obtained at high L/W leads to decreases in the unbleached pulp yield and viscosity, agreeing with the results obtained for AA variations (Table 1). The observed dependency of the bleaching response on the pulping parameters for the overall bleaching sequence (D0E1D1E2D2) was investigated separately for the D0E1 and D1E2D2 stages. In the case of the series of pulps produced with different sulfidities, the bleaching response in the first chlorine dioxide stage (including alkaline extraction, D0E1), where the kappa number was reduced from 14 to 6, was shown to be almost independent from the pulping conditions (Figure 4). However, in the final two chlorine dioxide stages, the bleachability is strongly affected by the pulping conditions (Figure 4). Thus, the observed dependency of bleachability on sulfidity for the overall bleaching sequence (Figure 2) is determined essentially by the bleaching response of the pulp in the D1E2D2 stages. The
same behavior was observed for the series of pulps produced with different AA values and liquor-to-wood ratios. In the case of unbleached pulps with different kappa numbers (see first part of Results and Discussion), the observed differences in bleachability behavior could be correlated with lignin chemical features. However, for unbleached pulps with the same kappa number, the establishment of a cause-and-effect relationship is not an easy task14,15 because of the complexity associated with the effect of process variables on different pulp components (namely, those consuming chlorine dioxide during bleaching, i.e., lignin, hexenuronic acid, and extractives) and with the small variations of pulp chemical features and limitations of available extraction and analytical techniques. This subject will be discussed separately in forthcoming papers. Variation of Alkali and Temperature Profiles. The alkaline profile inside a kraft pulping digester is claimed to affect the yield and properties of unbleached pulp significantly.16 To investigate the effect on bleachability of different active alkali distributions in the course of the cooking, a series of kraft pulps was obtained by splitting the alkaline charge into two parts: the first part (for example, 35% of total active alkali) was introduced in the beginning of the cooking and the second part (65% of total active alkali) was introduced at end of the warm-up period, when the reactor reached the final cooking temperature (160 °C). Three combinations were tested: 35%/65%, 65%/35%, and 80%/20%. The temperature profile was also slightly modified: two sets of pulps (with different alkaline splitting schemes) were produced by varying the initial temperature of the pulping liquor in the beginning of the cooking stage: 40 °C (reference) and 72 °C. The pulping time was adjusted to give a constant kappa number of 14.0 ( 0.4. All other pulping parameters were kept constant at their reference values (Table 2). When the initial pulping liquor was introduced into the reactor at 40 °C, the splitting of the alkaline charge did not have a noticeable effect on the bleaching response of the kraft pulps (Figure 5). However, surprisingly, when the cooking was performed with pulping liquor preheated to 72°, the splitting of the alkaline charge into two fractions had a detrimental effect on the bleachability (Figure 5). This was particularly pronounced in the case where only a small fraction of alkali was introduced in the beginning of the cooking stage, and the remaining alkali was added later (35%/ 65%) when the final cooking temperature (160 C°) was reached. This is probably related to the fast depletion of active alkali (as indicated by a residual alkali near 0) in the pulping solution, in the early stages of delignification, which hinders the progress of delignification. However, at this stage of our investigation, we do not have a clear explanation for the negative impact of alkali depletion on bleachability. Variations in the alkaline profile and temperature profile during the cooking did not show any clear effect on pulp yield or polysaccharide degradation, as indicated by the relatively small variations in pulp viscosity (Table 2). Pre-extraction of Wood Chips. Wood extractives include low-molecular-weight compounds bearing unsaturated moieties or functionalities,1 which can undergo oxidation reactions. Part of these compounds is carried to the bleaching plant and can react with
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Table 2. Conditions and Results of Modified Kraft Cookingsa modifications temperature of white liquor addition (°C)
splitting of alkaline charge (%)
time (min)
pulp yield (%)
kappa number
viscosity (mL/g)
brightness (%)
ClO2 consumption (%)
40 40 40 40 72 72 72
100 35/65 65/35 80/20 100 35/65 65/35
170 188 172 170 131 136 150
55.5 56.0 56.0 56.1 56.0 55.5 55.3
13.8 13.9 14.3 13.6 14.0 13.8 14.2
1387 1440 1470 1444 1367 1386 1442
47.8 44.9 45.4 47.2 44.8 44.8 45.7
4.10 4.11 4.12 3.97 4.14 4.46 4.33
wood chip pre-extraction 55.5 13.8 54.7 14.2 51.9 14.2
1387 1353 1351
47.8 52.8 46.9
4.10 3.80 4.29
no extraction 0.3% NaOH aqueous solution ethanol/water, 2:1 a
170 148 160
T ) 160 °C, AA ) 17%, S ) 28%, L/W ) 4.
Figure 5. Influence of the alkaline charge splitting and temperature of white liquor addition on bleachability by a D0E1D1E2D2 bleaching sequence (90.5% brightness). (See Table 2 for conditions.)
Figure 6. Influence of wood chip pre-extraction on pulp bleachability by a D0E1D1E2D2 bleaching sequence. (See Table 2 for conditions.)
chlorine dioxide, leading to the consumption of chemicals in “nonbleaching” reactions.17 To assess the influence of wood extractives on kraft pulp bleachability, two cooks were carried out using wood chips previously preextracted with aqueous 0.3% NaOH solution (100 °C, 4 h, liquor-to-wood ratio of 4) or ethanol-water 2:1 (v/v) (80 °C, 6 h, liquor-to-wood ratio of 4). The yields of extracts were, 8.0 and 2.1% (odw), respectively. The pulping time of washed and dried pre-extracted chips was adjusted to give a kappa number of 14.0 ( 0.2. The other pulping parameters were kept constant at their reference values (Table 2). As shown in Figure 6, the alkaline wood pre-extraction significantly improves the bleaching response of pulp. The alkaline pre-extraction removes lipohilic and
polyphenolic extractives, namely, hydrolyzable and condensed tannins, which are quite abundant in Eucalyptus species,1 and a small fraction of alkali-soluble lignin.18 The bleachability improvement can be assigned, at least partially, to the extraction of condensed tannins during the alkaline pretreatment, reducing the possibility of chromophore structure formation during pulping. This explanation is supported by the higher brightness of the pulp obtained. In fact, this pulp shows the highest brightness of all unbleached pulps produced in this work. Additionally, when an alkaline preextraction is carried out, the overall alkaline charge used in the pulping (pre-extraction included) leads to a higher residual alkali level at the end of the cooking stage, thus improving the bleachability, in agreement with the results shown above. The ethanol/water pre-extraction leads to a decrease of pulp bleachability (Figure 6). Simultaneously, the unbleached pulp yield is reduced (6.5%), when compared to reference pulp yield (Table 2). At present, we do not have a clear explanation for this negative impact on bleachability and pulp yield. Bleachability and Brightness of Unbleached Pulps. It might be expected that unbleached pulps with higher brightness would be easier to bleach. In fact, this is observed when the active alkali is increased, while all other pulping parameters are kept constant at their reference values (Table 1). However, this is not the case when the other pulping parameters, particularly sulfidity and liquor-to-wood ratio, are considered: brightness is not significantly affected (Table 1), whereas
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Figure 7. Relationship between the brightness of unbleached pulp and partially bleached (D0E1) pulp and bleachability by a D0E1D1E2D2 bleaching sequence (90.5% brightness).
bleachability can vary over a wide range (Figure 2). Globally, no sound relationship can be established between unbleached pulp brightness and pulp bleaching response in an ECF sequence (Figure 7). Thus, the brightness of the unbleached pulp cannot be used to predict the bleaching response in an ECF sequence. When the brightness of partially bleached (D0E1) pulps (kappa number 6) is considered, the same general behavior is observed (Figure 7), particularly when the pulp issued from a cooking stage carried out at L/W ) 8 (out of the range used in industrial practice), which showed the lowest bleachability [96.5 (OXE/ton)/kappa number], is not considered in the analysis. Bleachability and H Factor. As previously seen (Figure 2), the bleachability has a complex dependence on the pulping parameters used to produce the pulp. It would be interesting, from the point of view of industrial practice, to identify a single variable that could be related to bleachability and used in the pulp mill to control chlorine dioxide consumption in the bleaching plant. Vroom19 proposed a simple kinetic model in which time (t) and temperature (T) are combined in a single variable in the form of the time integral of the relative rate constant (kr ) k/k373, where k is the rate constant at a given temperature and k373 is the rate constant at 373 K), the so-called H factor
H)
∫kr dt ) ∫exp(43.2 - 16.1/T) dt
Because the extent of delignification is proportional to the H factor, this model is widely used to estimate the time and temperature required to attain a desired kappa number in an industrial digester. AA, S, and L/W affect the pulping time, and thus, the H factor reflects the variations of all pulping parameters investigated in this work. When the bleachability of kraft pulps with kappa number 14, obtained by variations of AA, S, T, and L/W (Table 1), expressed as (OXE/ton)/kappa number is plotted against the H factor, an interesting tendency is observed: the bleachability increases as the H factor decreases (Figure 8). Tentatively, this result can be assigned to the influence on bleachability of side reactions occurring between lignin or its degradation products and polysaccharides in fibers during the pulping, leading to the establishment of new ligninpolysaccharide linkages.20 As the H factor is increased (particularly by extending the pulping time), the probability of occurrence of such reactions will increase, leading to poorer lignin reactivity in the bleaching. From the point of view of bleachability, ECF-bleached kraft
Figure 8. Relationship between ECF bleachability (90.5% brightness) and H factor (unbleached pulps with kappa number 14 ( 0.3, obtained by variation of AA, S, T, L/W; see Table 1 for conditions).
pulp mills should work with an H factor as low as possible. Low H factors can be attained by using high active alkali and/or high sulfidity [pulping time at a given temperature, is reduced (Table 1)]. However, high active alkali levels lead to significant pulp yield and pulp viscosity losses (Table 1). On the other hand, high sulfidity levels lead to yield improvements and to better pulp quality (higher viscosity) but to additional problems in the mill, namely, equipment corrosion and increased sulfur-based emissions. Optimization of these factors in each particular mill, using H factor control, can lead to substantial economic and environmental impact savings. Conclusions Pulping parameters, namely, active alkali, sulfidity, temperature, and liquor-to-wood ratio, significantly affect the bleachability of Eucalyptus globulus kraft pulps bleached by a conventional ECF (DEDED) bleaching sequence. The best pulp bleaching response, pulp yield, and pulp viscosity are obtained for a high sulfidity level (37%), with all other pulping parameters kept constant at standard levels. Splitting of the alkaline charge during pulping, combined with variations in the temperature profile, in general, adversely affects bleachability, whereas pre-extraction of the wood with diluted NaOH solution significantly improves the bleachability of the pulp. The extent of delignification of the unbleached pulp (kappa number) affects the bleaching response of the pulp, with the residual lignin and other oxidizable structures in the pulp being progressively more difficult to remove/bleach by ClO2 as their content in the pulp decreases. No relationship could be established between unbleached pulp brightness and bleachability. Bleachability can be related to the H factor used in the pulping. Low H factors, obtained by variations in other parameters apart from temperature, afford pulps that are easier to bleach. This result might have a direct impact on the control strategy of chlorine dioxide consumption in an ECF bleached kraft pulp mill. The variations observed in bleachability can be related to modifications in the structure and composition of pulps induced by the different pulping conditions. This topic will be discussed in detail in future papers. Acknowledgment The authors thank the European Commission for the financial support of the research project FAIR CT98 3460 within which this work was carried out. The authors are also grateful to Ana Isabel Cavaleiro,
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Sandra Matos, and Carla Gonc¸ alves, the research technicians involved in the pulping and bleaching experiments. Literature Cited (1) Sjo¨stro¨m, E. Wood Chemistry. Fundamentals and Applications, 2nd ed; Academic Press: New Tork, 1993. (2) Dence, C. W.; Reeve, D. W. Pulp Bleaching. Principles and Practices; TAPPI Press: Atlanta, GA, 1996. (3) Gustavson, C.; Sjo¨stro¨m, K.; Wafa Al-Dajani, W. The influence of cooking conditions on the bleachability and chemical structures of kraft pulps. Nord. Pulp Pap. Res. 1999, 14, 71. (4) Gellerstedt, G.; Wafa Al-Dajani, W. The effect of cooking parameters on the chemical properties and bleachability of alkaline pulps. In Proceedings of the International Pulp Bleaching Conference; PAPTAC: Montreal, Canada, 2000; p 37. (5) Blain, T. J. The influence of sulfidity on the bleachability and strength properties of alkaline-anthraquinone softwood pulps. Tappi J. 1980, 63, 125. (6) Kumar, K. R.; Chang, H. M.; Jameel, H. Effect of pulping conditions on the bleachability of hardwoods. In TAPPI Pulping Conference, Proceedings; TAPPI Press: Atlanta, GA, 1995; Book 2, p 539. (7) Rawat, N.; McDonough, T. J. Effects of pulping conditions on the bleachability of hardwood kraft pulps. 1. Effects of effective alkali charge in the pulping of birch and maple. In Proceedings of the TAPPI Pulping Conference; TAPPI Press: Atlanta, GA, 1998; Part 2, p 883. (8) Sjo¨strom, K. Kraft cooking with varying alkali concentrations Influence on TCF bleaching. Nord. Pulp Pap. Res. 1998, 13, 57. (9) Carvalho, M. G. V.; Saleiro, S. I. C.; Martins, A. A.; Figueiredo, M. M. L. Branqueabilidade de pastas kraft de E. Globulus. In Livro de Resumos do 16° Encontro Tecnicelpa; Universidade da Beira Interior: Covilha˜, Portugal, 1998; p 382. (10) Colodette, J. L.; Gomide, J. L.; Girard, R.; Jaaskelainen, A. S.; Argyropoulos, D. S. Influence of pulping conditions on eucalyptus kraft pulp yield, quality and bleachability. Tappi J. 2002, 1, 14. (11) Grundelius, R. Oxidation equivalents, OXEsAn alternative to active chlorine. In Proceedings of the International Pulp Bleaching Conference; TAPPI Press: Atlanta, GA, 1991; p 49. (12) TAPPI Test Methods 1996-1997, TAPPI Press: Atlanta, GA, 1996. (13) Pascoal Neto, C.; Daniel, A. I. D.; Evtuguin, D.; Silvestre, A. J. D.; Furtado, F. P.; Cavaleiro, A. I.; Sousa, A. P. M. Influence
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Received for review April 8, 2002 Revised manuscript received September 11, 2002 Resubmitted for review June 25, 2002 Accepted September 16, 2002 IE020263X
