The distribution of oxidizable structures in ECF- and TCF-bleached kraft pulps

The distribution of oxidizable structures in ECFand TCF-bleached kraft pulps Jiebing Li, Olena Sevastyanova and Göran Gellerstedt, Royal Institute of Technology, Stockholm, Sweden Keywords: Bleaching, Chlorine dioxide, Hexenuronic acid, Hydrogen peroxide, Kappa number, Kraft pulps, Lignin, Oxygen, Permanganate consumption SUMMARY: The kappa number in kraft pulp contains contributions from lignin and from other oxidizable structures denoted as false lignin. The latter can be divided in hexenuronic acid groups and ”non-lignin” structures. In this paper, the kappa number units due to the various contributing structures have been quantified by fractionation of the kappa number. Bleached spruce and birch kraft industrial pulps taken after each stage in an ECF (ODEQP) and in a TCF (OQOPQPO) bleaching sequence respectively have been assessed. Possible correlations between the content of false lignin, the pulp origin, the bleaching conditions, and brightness and viscosity values have been sought. It is concluded that the false lignin structures are formed predominantly during the cooking and oxygen delignification stages. The false lignin structures were found to give different responses in the various bleaching stages, but to have no influence on either pulp brightness or viscosity. ADDRESS OF THE AUTHORS: J. Li, O. Sevastyanova, G. Gellerstedt. Department of Pulp and Paper Chemistry and Technology, Royal Institute of Technology, SE-100 44 Stockholm, Sweden The content of residual lignin in a chemical pulp is an important parameter for process control in both pulping and bleaching. In practice, the lignin content is most commonly determined using the kappa number method [SCAN-C1:77, 1977]. At the same time, it is known that the relationship between kappa number and lignin content is not straightforward and varies depending on wood species and pulping process. The lignin accounts for only a portion of the total kappa number which reflects the content of all types of oxidizable structures present in the pulp (Li, Gellerstedt 2002). Such oxidizable structures, denoted as false lignin or pseudo-lignin, have been shown to be present in unbleached kraft pulps. These structures can be divided into two groups, viz. hexenuronic acid (HexA) groups and ”non-lignin” structures. The exact chemical structure(s) of the latter are not known but they are presumably of carbohydrate origin and may contain double bonds, aldehyde and α-keto-carboxyl groups as possible functional groups (Li et al. 2002). Like lignin, the false lignin consumes bleaching chemicals and will presumably add to the environmental load of the bleaching effluent. It has, for example, been demonstrated that HexA on treatment with chlorine dioxide forms chlorinated aliphatic acids (Vuorinen et al. 1997). In addition, the false lignin may influence the pulp properties. Therefore, an understanding of the mechanism of formation of the false lignin and its chemical properties as well as a means of quantification in different types of kraft pulps are of importance. In a previous paper, different unbleached pulps were investigated in order to assess the relative contributions of Nordic Pulp and Paper Research Journal Vol 17 no. 4/2002 lignin, HexA and ”non-lignin” structures to the kappa number (Li, Gellerstedt 2002). Work has also been done to determine the amounts of permanganate consumed by lignin in the kappa number determination in unbleached as well as in different types of bleached kraft pulps from spruce and birch (Li et al. 2002). It was found that bleaching reactions lead to a decrease in the molar consumption of permanganate due to a partial oxidation of aromatic rings by the bleaching agent. In order to quantify the amount of lignin present in a chemical pulp, whether unbleached or bleached, a modified procedure for kappa number determination has been developed. This is based on the selective removal of HexA as well as of those ”nonlignin” structures which contribute to the kappa number as measured by the SCAN-test or Tappi method (Li, Gellerstedt 2002). The remaining oxidizable material, i.e. lignin, present in the pulp is quantified using permanganate oxidation. In the present paper, two industrial pulps, an ECF-bleached spruce pulp and a TCF-bleached birch pulp, have been analysed after each stage in the bleaching sequence in order to quantify the amounts of lignin, HexA and ”nonlignin” structures that remain in the pulp as the delignification and brightening proceeds. Attempts have also been made to correlate these amounts to pulp brightness and viscosity. Experimental Materials Samples of unbleached and bleached spruce and birch pulps from an ODEQP and an OQ(OP)Q(PO) sequence respectively were taken from a Swedish mill after each bleaching stage at time intervals corresponding to the retention time characteristics of each stage. From each position, two different samples were taken and mixed together, homogenised, thoroughly washed with deionized water, and air dried. Analytical determinations The standard kappa number and the Ox-Dem kappa number were determined according to [SCAN-C1:77] and (Li, Gellerstedt 2002), respectively. The typical experimental error in the kappa number determination is ±0.1 kappa unit. Hexenuronic acid was quantified using the colorimetric method described in (Gellerstedt, Li 1996). Klason lignin was quantified by the TAPPI Test Method [T222 om83]. Carbohydrate analysis was carried out using acid hydrolysis and gas chromatography as described by Theander and Westerlund (1986). Viscosity and brightness measurements were measured according to [SCAN-CM 15:99] and [SCAN-P3:93], respectively. 415 Results and discussion the kappa number of the ”non-lignin” structures is calculated as the difference between the standard pulp kappa number and the sum of the contributions of the residual lignin and HexA. In this way, the kappa numbers of different bleached kraft pulps have been fractionated into the contributions of the residual lignin, the HexA and the ”non-lignin” structures, as shown in Table 1; the contribution of residual lignin being determined as the Ox-Dem kappa number. Fractionation of the kappa number in bleached kraft pulps Regardless of the pulp origin and bleaching history, the pulp kappa number is always the sum of contributions from all the permanganate-oxidizable structures present in the pulp, including lignin and false lignin. False lignin can be further divided into two groups comprising hexenuronic acid groups and other ”non-lignin” structures. It should be kept in mind that each individual contributor can undergo changes, both in chemical structure and in amount, during the bleaching operations. The contribution to the kappa number of the true residual lignin can be estimated from the Klason lignin content, provided that the permanganate oxidation equivalent of lignin is known. For unbleached pulps, the oxidation equivalent of lignin is about 11.6 per phenylpropane (C9) unit (Li, Gellerstedt 1998). Since lignin undergoes successive oxidative degradation reactions during the course of bleaching, its molar oxidation equivalent decreases, however, and it must consequently be determined after each bleaching stage for the above method to be applicable. Alternatively, the contribution of the lignin can be measured directly using the Ox-Dem kappa number method. It has been demonstrated that the Ox-Dem kappa number correctly reflects the true residual lignin content in bleached pulps even when the lignin structure has been significantly degraded by oxidative treatment (Li et al. 2002). In a bleaching sequence, the hexenuronic acid (HexA) groups can be more or less efficiently removed from the pulp depending on the bleaching agents used in the sequence. The contribution of the remaining HexA to the kappa number can be readily calculated after measuring its content in the pulp by any available method (Gellerstedt, Li 1996; Teleman et al. 1995; Vuorinen et al. 1999). The molar oxidation equivalent of HexA in the kappa number determination has been found to be 8.6 (Li, Gellerstedt 1997). As in the case of unbleached pulps, the contribution to Formation and changes in false lignin in bleached pulps Fractionation of the pulp kappa number enables the changes in the amounts of HexA and ”non-lignin” structures to be followed as the bleaching sequence proceeds. In Table 1, it can be seen that HexA is present in the pulp after the cook and then subsides during the oxygen delignification and subsequent bleaching stages. The actual content of HexA after each stage depends on the wood species and on the type of bleaching operation applied (Buchert et al. 1995; Teleman et al. 1995). HexA is present in a smaller amount in the ECF-bleached spruce kraft pulp than in the TCF-bleached birch kraft pulp, and it is eliminated mainly by chlorine dioxide . In the bleached birch kraft pulp, HexA appears to be the major contributor to the kappa number and its contribution remains significant throughout the entire bleaching sequence. These results confirm the well-known fact that HexA is not eliminated by the action of either oxygen or hydrogen peroxide. ”Non-lignin” structures are present in the pulp after the kraft cook and in the subsequent oxygen-delignification stage. In the spruce kraft pulp, the amount of such structures was found to correspond to 4.0 kappa units after the cook and to increase to 4.9 kappa units after the oxygen delignification. The ECF-sequence was efficient in removing the ”non-lignin” structures and, at the end of the sequence, only trace amounts were still present in the pulp. The contribution of the ”non-lignin” structures was found to be smaller in the unbleached birch Table 1. Fractionation of pulp kappa number of spruce and birch kraft pulps bleached kraft pulp than in the spruce pulp and according to the ODEQP and OQ(OP)Q(PO) sequence respectively. corresponded to ~1 kappa unit. After Pulp Pulp kappa number Ox-Dem kappa number Kappa number from false lignin the oxygen stage, this contribution from residual lignin Kappa number Kappa number from HexA* from "non-lignin" increased to about 2 kappa units folstructures** lowed by a steady decrease in the perSpruce kraft pulp oxide stages. In contrast to the ECFUnbleached 22.5 17.2 1.3 4.0 sequence, a noticeable amount of O-delignified 10.7 4.6 1.2 4.9 ”non-lignin” structures was, however, OD-bleached 5.2 2.3 0.8 2.1 still present in the fully bleached ODE-bleached 3.3 1.6 0.8 0.9 pulp. ODEQ-bleached 2.3 1.4 0.7 0.2 ODEQP-bleached 1.6 0.8 0.7 0.1 Birch kraft pulp Unbleached O-delignified OQ-bleached OQ(OP)-bleached OQ(OP)Q-bleached OQ(OP)Q(PO)-bleached 13.8 9.6 9.4 7.3 7.0 4.6 8.1 3.2 3.0 2.2 1.9 0.8 4.7 4.3 4.3 3.7 3.7 3.0 1.0 2.1 2.1 1.4 1.4 0.8 * Calculated from the content of HexA analysed according to (Gellerstedt, Li 1996) and using the molar equivalent of 8.6 taken from (Li, Gellerstedt 1997) ** Calculated by subtracting the Ox-Dem kappa number and the HexA kappa number from the pulp kappa number 416 Bleaching response of false lignin The false lignin contains oxidizable structures that can be attacked by various bleaching chemicals. By comparing the amount of false lignin before and after each bleaching stage, the efficiency of a given operation with respect to the removal of false lignin can be judged. Furthermore, by fractionation of the kappa number as Nordic Pulp and Paper Research Journal Vol 17 no. 4/2002 Fig.1 Residual lignin, HexA and ”non-lignin’’ structures in spruce kraft pulps bleached according to the ODEQP sequence. Fig.2 Residual lignin, HexA and ”non-lignin’’ structures in birch kraft pulps bleached according to the OQ(OP)Q(PO) sequence. Fig. 3. The ratio of HexA content to xylan content in (a) spruce and (b) birch kraft pulps bleached according to the ODEQP and OQ(OP)Q(PO) sequences respectively. Nordic Pulp and Paper Research Journal Vol 17 no. 4/2002 described above, further information about differences in the bleaching response between true residual lignin and false lignin components, including HexA and ”non-lignin” structures, can be obtained. The bleaching response, expressed as the percentage reduction in the kappa number of each of the lignin, HexA and ”non-lignin” structures is shown in Figs 1 and 2 for the spruce and birch pulps respectively. Here, it can be seen that oxygen is an efficient delignification agent for both softwood and hardwood pulps. Although the normal kappa numbers indicate only about 52 and 30% reduction respectively, the degree of lignin dissolution is, in fact, about 73 and 60%. In the further bleaching of the pulps, both chlorine dioxide (together with a subsequent extraction stage) and alkaline hydrogen peroxide are rather efficient in removing lignin, and the remaining amount of lignin in the fully bleached pulps is in both cases around 0.8, determined as Ox-Dem kappa number. In the case of the two pulps employed in this work, however, the removal of lignin seems to be much more efficient in the softwood case, since both oxygen and chlorine dioxide oxidation led to a greater lignin removal than in the hardwood case in which two oxygen stages (the second with peroxide reinforcement) were employed. In the former case, a total amount of lignin corresponding to 15.6 kappa units was eliminated whereas from the birch pulp, only 5.9 kappa units were removed. Therefore, the softwood pulp can be subjected to a milder final peroxide bleaching stage than the hardwood pulp in order to reach full brightness. Neither of the bleaching sequences was able to remove any large amount of HexA. Since a softwood pulp contains much less HexA after the cook than a hardwood pulp, the contribution to the kappa number of HexA in the softwood pulp, both before and after the bleaching was, however, much less. In the softwood pulp, a certain elimination of HexA took place in the chlorine dioxide stage since the HexA/xylan ratio decreased (Fig. 3). This is in agreement with earlier observations (Tenkanen et al. 1999; Ragnar 2001). The large amount of HexA present in the unbleached birch pulp gradually decreased under the alkaline conditions prevailing in the bleaching sequence which presumably lead to an alkaline extraction of a portion of the HexA-bound xylan. A direct oxidative elimination of HexA does not take place with either oxygen or hydrogen peroxide (Tenkanen et al. 1999) The amount of the other part of the false lignin, the ”non-lignin” structures, increased somewhat during the oxygen-delignification stage. In the subsequent bleaching stages with either chlorine dioxide or hydrogen peroxide, a gradual decrease took place, the chlorine dioxide and the subsequent extraction stages being the most efficient. Influence of false lignin on pulp properties The main purpose of bleaching is to obtain a clean pulp with high brightness. Traditionally, the residual lignin present in the pulp after the cook has been regarded as the detrimental component, although no distinction has been made between lignin and other possible contaminants in the pulp. In order to find out the relative contributions of 417 Gellerstedt 2002). The hexenuronic acid group, formed by elimination of methanol during the kraft cook, conBrightness Viscosity tains a double bond but on permang(% ISO) (dm3/kg) anate oxidation it degrades in such a way that new oxidizable functional 28.9 1080 groups are formed. Consequently, the 44.7 910 oxidation equivalent for this structure 61.5 850 is high (Li, Gellerstedt 1997). The 73.3 850 73.3 850 ”non-lignin” structures are also for88.8 800 med during the kraft cook and, to a lesser extent, during the oxygen41.0 1140 delignification stage. Thus, alkaline 59.1 920 conditions seem to be a prerequisite 59.3 870 and, again, elimination reactions may 73.3 850 be involved. The most probable ori73.5 790 89.0 710 gin of these structures is carbohydrate moieties which undergo elimination of water or, alternatively, of a sugar unit. Such reactions have been demonstrated using model compounds, the final products being aromatic structures containing catechols and chromones (Forsskåhl et al. 1976). In a polysaccharide structure, a more ”diffuse” distribution of functional groups originating from elimination reactions may, however, result in keto groups in equilibrium with the corresponding enol structures and located along the polysaccharide chain in e.g. hemicelluloses. Such groups will consume permanganate in a kappa number determination and an extra quantity of bleaching agent in bleaching. The reactivity towards permanganate as well as to various bleaching agents, will differ depending on the predominance of the keto or the enol form. As with HexA, the ”non-lignin” structures are more predominant in the birch pulp, bleached with only oxygen and hydrogen peroxide. The suggested chemical similarity between the two components of false lignin may therefore mean that they are both involved in e.g. the thermal yellowing of bleached hardwood pulps (Granström et al. 2001). Table 2. Carbohydrate composition, brightness and viscosity of spruce and birch kraft pulps bleached according to the ODEQP and OQ(OP)Q(PO) sequences respectively. Pulp Glu Spruce kraft pulp Unbleached O-delignified OD-bleached ODE-bleached ODEQ-bleached ODEQP-bleached Birch kraft pulp Unbleached O-delignified OQ-bleached OQ(OP)-bleached OQ(OP)Q-bleached OQ(OP)Q(PO)-bleached Carbohydrate Composition (%) Xyl Ara Man Gal 75.5 76.2 76.5 76.3 76.8 76.6 12.3 9.7 9.5 9.7 10.2 9.3 1.0 1.1 1.0 1.2 1.1 1.1 5.9 5.8 5.7 5.8 5.7 5.7 0.5 0.2 0.2 0.3 0.3 0.3 66.4 68.4 67.1 24.8 24.3 24.2 0.9 0.4 0.3 0.6 0.6 0.6 0 0 0 67.2 69.8 69.3 24.1 23.8 23.1 0.3 0.8 0.4 0.5 0.7 0.5 0 0 0 lignin and false lignin to pulp properties such as brightness and viscosity, these values were measured on all the pulp samples employed in this work. The data are listed in Table 2. In Figs. 4 and 5, the relationship between the brightness and the amounts of lignin, HexA and ”non-lignin” structures expressed in kappa number units are shown. There is clearly a good correlation between brightness and pulp lignin content irrespective of wood species and bleaching sequence whereas no such correlation can be found for either HexA or ”non-lignin” structures. In a similar way, correlations can be found between the lignin kappa number and the viscosity (Figs. 6 and 7). Here, however, the pulps behave differently and, as expected, the TCF-sequence gives a stronger decrease in viscosity. Again, neither the HexA nor the ”non-lignin” structures seem to correlate with the viscosity during bleaching. ”Non-lignin” structures In earlier work, it was shown that in addition to lignin, major contributions to the kappa number are given by functional groups such as double bonds, aldehydes or αketo-carboxyl groups and combinations of these (Li, Fig.4 Brightness vs. Ox-Dem kappa number for spruce and birch kraft pulps bleached according to the ODEQP and Q(OP)Q(PO) sequence respectively. 418 Fig. 5 Pulp brightness plotted against the amount of HexA and other non-lignin structures i spruce and birch kraft pulps bleached according to the ODEQP and OQ(OP)Q(PO) sequences respectively. Nordic Pulp and Paper Research Journal Vol 17 no. 4/2002 Lignin, on the other hand, is efficiently removed by either oxygen, hydrogen peroxide or chlorine dioxide and only a very small amount remains in the bleached pulp. Acknowledgement Financial support to one of us (OS) from the Jacob Wallenberg Research Foundation is gratefully acknowledged. This work is a part of the ”Ecocyclic Pulp Mill” research program financed by MISTRA, the Swedish Foundation for Strategic Environmental Research. Literature Fig. 6 Pulp viscosity plotted against Ox-Dem kappa number of spruce and birch kraft pulps bleached according to the ODEQP and OQ(OP)Q(PO) sequences respectively. Fig.7 Pulp viscosity plotted against the amount of HexA and other non-lignin structures in spruce and birch kraft pulps bleached according to the ODEQP and OQ(OP)Q(PO) sequences respectively. Conclusions The kappa number in unbleached and bleached kraft pulps can be divided in contributions of lignin, hexenuronic acid groups and ”non-lignin” structures, the latter two constituting false lignin. The predominant portion of the false lignin is present in the pulp after the cook but additional ”non-lignin” structures are formed in the oxygen delignification stage. During bleaching, both brightness and viscosity development are closely related to the true lignin content of the pulp, whereas the false lignin does not show any similar systematic changes. Both hexenuronic acid and the ”nonlignin” structures are partly removed by a chlorine dioxide stage, but the false lignin may contribute considerably to the residual kappa number in the fully bleached pulp. Nordic Pulp and Paper Research Journal Vol 17 no. 4/2002 Buchert, J., Teleman, A, Harjunpää, V., Tenkanen, M., Viikari, L. and Vuorinen, T. (1995): Effect of cooking and bleaching on the structure of xylan in conventional pine kraft pulp. Tappi J. 78:11, 125-130. Forsskåhl, I., Popoff, T. and Theander, O. (1976): Reactions of Dxylose and D-glucose in alkaline, aqueous solutions. Carbohydr. Res. 48, 13-21. 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(1997): Selective hydrolysis of hexenuronic acid groups opens new possibilities for development of bleaching processes. 9th International Symposium on Wood and Pulping Chemistry, Montreal. Proceedings, Oral Presentations M4. Vuorinen, T., Fagerström, P., Buchert, J., Tenkanen, M., Teleman, A. (1999): Selective hydrolysis of hexenuronic acid groups and its application in ECF and TCF bleaching of kraft pulps. J. Pulp Pap. Sci. 25, 155-162. Manuscript received June 12, 2002 Accepted August 2002 419