REMOVAL OF NON-PROCESS ELEMENTS FROM HARDWOOD CHIPS
PRIOR TO KRAFT COOKING

Fredrik Lundqvist, Harald Brelid, Anna Saltberg, Göran Gellerstedt and Per Tomani

Presented at the 59th Appita Conference, 16-19 May 2005, Auckland, New Zealand

ABSTRACT

The major part of the intake of non-process elements (NPE's) to the pulp mill is via the wood chips. Some of the problems associated with NPE's are e.g. precipitation of sparingly soluble calcium salts in the fibre line and in the recovery department. In order to investigate the possibilities of reducing the intake of NPE's via hardwood chips to the pulp mill, laboratory studies on the acidic leaching of birch and eucalyptus chips were carried out. The results showed that potassium, magnesium and manganese were removed from both wood species at similar rates. The removal of calcium was however significantly slower from eucalyptus than from birch.

Removal of NPE's from birch wood chips prior to cooking resulted in a higher rate of delignification, a higher unbleached brightness and a higher viscosity. In the case of eucalyptus, acidic leaching had no effect on the rate of delignification. The positive effect of acidic leaching of birch chips was found to be due to a lower content of calcium in the cooking stage.

INTRODUCTION

Most of the intake of non-process elements (NPE's) to the kraft process usually is via the wood chips. Depending on the wood raw material, degree of recycling of filtrate streams and the process technology applied, these NPE's may cause severe production related problems in the pulp mill (Ulmgren 1997). In general, the most problematic NPE is calcium, because of its tendency to precipitate in presence of carbonate. Calcium carbonate has a propensity to form supersaturated solutions and it has a reversed solubility (the solubility decreases as the temperature is increased). Accordingly, calcium has a strong tendency to form carbonate scales on hot surfaces, e.g. in heat exchangers and in black liquor evaporators (Hartler and Libert 1973, Westervelt et al. 1982, Lidén et al. 1996, Werner et al. 1998).

In the bleach plant, calcium may also precipitate as sparingly soluble calcium oxalate and cause clogging of e.g. washer drums and leading to a reduced production capacity. Problems related to calcium oxalate precipitation predominantly occur under acidic or neutral conditions (Ulmgren and Radestrom 1997). Other wood originating NPE's may also cause process disturbances in the pulp mill and a well-known example is the detrimental effects of transition metals on peroxide bleaching (Lachenal 1996).

Metal ions in wood are, to a large extent, present as counter ions to carboxylate groups in the non-cellulosic carbohydrates. A large part of the metal ions can be removed by subjecting the wood material to an acidic leaching (Brelid et al. 1998). Introducing an NPE-removing leaching of the wood chips prior to pulping can decrease the problems associated with NPE's without any negative effects on pulp quality, thus enabling a higher degree of recycling of bleach plant filtrates to the recovery area (Lindgren et al. 2002, Axegard 2003).

Removal of metal ions from wood chips has been reported to gain additional positive effects in the fibre line besides the benefit of reducing NPE related problems. For example, chip leaching yields a higher unbleached pulp brightness and a somewhat higher rate of delignification in pulping of softwoods (Brelid et al. 1998, Gellerstedt and Al-Dajani 1998, Brelid 2002). A study of the effects of a restoration of the transition metal ion profile after acidic leaching of softwood chips prior to kraft pulping further indicates that the metal ion profile in chips influences the delignification (Kangas et al. 2002).

In the present study, the chip-leaching concept has been applied to hardwood chips prior to kraft pulping. The study is focused on birch (Betula pendula) but also includes some work on eucalyptus (E. globulus).

METHODS AND MATERIALS

Wood chips

In the initial leachings presented in this paper, industrial birch chips (mainly Betula pendula) from a Swedish pulp mill and eucalyptus chips (E. globulus) from a pulp mill in Portugal were used. The chips used were never dried. In the trials where the wood chips were subjected to cooking after leaching, the birch chips came from a 20-year birch log (Betula pendula) grown in the middle of Sweden. The log was chipped in a laboratory chipper at STFI-Packforsk and screened on a disc screen. The 2-8mm fraction was saved and prior to further use, bark and knots were removed by hand sorting. Industrial eucalyptus (E. Globulus) chips from Portugal were screened in accordance with the method described for the birch chips.

Leaching of wood chips

In the initial leaching trials, chips were placed in autoclaves and vacuum-impregnated with leaching liquor. The leaching was carried out at approx. pH 2.5, 80°C, different retention times and a liquor-to-wood ratio of 6:1. The desired pH was reached by an addition of sulphuric acid (3.25 kg/ton dry wood) to the leaching liquor. The same leaching conditions were used in the first trials where the wood chips were subjected to cooking after leaching. When the effects of an extensive metal ion removal and restoration of the metal ion profile in leached chips prior to cooking were studied, the chips were airdried before use. Prior to leaching, the chips were impregnated with water for 24 hrs. The acidic leaching of chips was performed at room temperature (22°C) for 24 hrs at a pH of 2.5 and a liquor-to-wood ratio of 10:1. The pH was adjusted with sulphuric acid. The leaching was carried out in covered plastic buckets with periodic stirring. After 6 h of leaching, the leaching liquor was replaced with fresh leaching liquor. The chips were then thoroughly washed with water and saved for further use.

Cooking and restoration of metals

Cooking was carried out by charging 100g (dry-weight) of wood chips in 1.0 L rotating stainless steel autoclaves in a pre-heated glycolic bath. All of the cooking chemicals were charged at the beginning together with the wood chips at a liquor-to-wood ratio of 4:1. In the case of cooking with restored magnesium or calcium, MgCl₂ or CaCl₂ was added to the white liquor. The EA-charge was 20% in the case of birch and 17% in the case of eucalyptus at a sulphidity of 35%. The temperature was raised from 80°C to 160°C at 1°C/min and maintained at 160°C for different times. The cook was terminated by cooling the autoclaves in a water bath prior to opening. Black liquor was collected and saved for further analysis. The cooked chips were washed for 10 hrs in running water. The chips were then defibrated in a water jet defibrator with 2 mm holes at a water pressure of 5 bars and subsequently screened on a Wennberg screen with 0.15 mm slits. Whenever applicable, deionised water was used. When the effects of restoration of metal ion content were studied, three cooking series were carried out according to Table 1.

Table 1. Description of cooking series.

The reference, denoted Ref, corresponds to cooking of water-impregnated wood chips. Cooking of acid-leached and acid-leached chips with restoration of either magnesium or calcium are denoted A, A-Mg and A-Ca respectively.

Analyses

Kappa number, viscosity and brightness were determined according to SCAN-test methods. Concentration of metals was determined by ICP-AES (inductively coupled plasma atomic emission spectroscopy). Prior to analysis, black liquors and leaching filtrates were wet-digested in nitric acid in a microwave oven in a closed vessel. Klason lignin, acid-soluble lignin and carbohydrates were analyzed according to STFI-method AH 23-18.

RESULTS AND DISCUSSION

Acidic leaching of birch and eucalyptus chips (initial trials)

Metal ion removal is promoted by low pH, high temperature and long retention time. Severe conditions during the leaching operation may cause acidic hydrolysis of the carbohydrates, which in a subsequent kraft pulping operation could result in a lower yield, and inferior strength properties of the pulp. However, previous studies of the leaching of Scandinavian softwood chips have shown that a substantial NPE-removal can be achieved when the leaching is performed at pH 2-3. At a leaching temperature not exceeding 100°C, the rate of acidic hydrolysis of carbohydrates is low and it has been shown that there are no negative effects on the properties of the subsequent kraft pulp (Axegard 2003). On the basis of these findings, the initial leachings of birch and eucalyptus chips in this study were carried out at approx. pH 2.5 and at a temperature of 80°C.

The leaching of calcium and potassium from industrial birch and eucalyptus wood chips is shown as a function of time in Figure 1. Under the conditions applied, the acidic groups in the wood will, to a large extent, be protonized, which leads to a liberation of metal ions from the wood matrix. However, the diffusion of ions from the interior of wood chips of industrial size is comparatively slow, because of the structure of the wood, which gives a hindered diffusion.

Figure 1 shows that potassium is leached faster than calcium from both eucalyptus and birch chips. The faster leaching rate of potassium can be explained by the higher diffusivity of potassium, but also by the fact that the monovalent potassium ions do not have such a high affinity for the fraction of the carboxylic groups in the wood polymers, which are still charged. In the case of potassium, the leaching rates from birch and eucalyptus were very similar. Calcium, on the other hand, was more difficult to remove from eucalyptus than from birch chips. This difference could be a consequence of differences in the morphological distribution of metal ions in the different wood species. But it may also be caused by a higher content of sparingly soluble calcium oxalate in the eucalyptus chips than in the birch chips. Results of a recent study indicate that a large part of the calcium present in eucalyptus wood can be in the form of calcium oxalate (Saltberg and Brelid 2005). When industrial eucalyptus wood chips were analyzed, it was found that 16% of the calcium was present in the form of calcium oxalate. In another eucalyptus wood sample, >50% of the calcium was shown to be in the form of calcium oxalate.

The latter value is probably not representative, but it indicates that there may be large variations in oxalate content in eucalyptus wood. In the case of industrial birch chips, about 11% of the calcium was found to be present as calcium oxalate.

Figure 1. Leaching of Ca and K from birch and eucalyptus chips. In the graphs, the amount of Ca and K remaining in the wood chips, respectively, are expressed as the ratio between the metal ion content at different times and the initial metal ion content in the chips (C/C0). Leaching conditions: Temperature 80°C, wood to liquor ratio 1:6, 3.25 kg H₂SO₄/ton dry wood chips and end pH about 2.5.

Eucalyptus wood is known to have a relatively high content of chloride and in this investigation, the untreated wood chips were found to contain 480 mg Cl^-/kg dry wood. In a kraft pulp mill, such a high chloride content in the wood raw material will, in the long term, lead to high concentrations of chloride in the recovery area. Since a high chloride concentration has a negative impact on the recovery cycle and complicates the control of the process, it is a great advantage if the intake of chloride is minimized. The results in Figure 2 show that the removal of chloride from wood chips is relatively rapid and that after 15 minutes of leaching about 43% of the original chloride remained in the chips.

Figure 2. Leaching of chloride from eucalyptus chips. In the graph, the amount of chloride remaining in the wood chips is expressed as the ratio between the chloride content at different times and the initial chloride content in the chips (C/C0). Leaching conditions: See Fig. 1.

Kraft pulping of leached birch and eucalyptus chips

Effect of acidic leaching

In a series of trials, kraft cooking of leached birch and eucalyptus chips was carried out. Figures 3 and 4 show the kappa number versus the H-factor for untreated and acid-leached birch and eucalyptus chips. Interestingly, acidic leaching of birch chips prior to cooking results in a significant increase in the rate of delignification, but it had no effect in the case of eucalyptus.

Figure 3. Kappa number vs H-factor for kraft cooking of untreated (Reference) and acidic leached (NPEkidney) birch chips. Leaching conditions: The same conditions as in Fig. 1 and the leaching time was 75 minutes.

Figure 4. Kappa number vs H-factor for kraft cooking of untreated (Reference) and acidic leached NPEkidney) eucalyptus chips. Leaching conditions: The same conditions as in Fig.1 and the leaching time was 75 min.

The significant effect on the rate of delignification of birch was studied further in a series of leaching and cooking trials. In order to more explicitly study the effect of the removal of NPE's from hardwood chips on pulping, birch and eucalyptus chips were acid-leached at room temperature at a pH of 2.5. The metal ion content in the birch and eucalyptus chips prior to and after leaching are presented in Tables 2 and 3. In the case of birch, the content of calcium decreased from 666 mg/kg to 166 mg/kg, i.e. a decrease of 75%. Magnesium and manganese were removed to approximately the same extent, whereas potassium was almost completely removed. As previously noted (cf. Figure 1) calcium was more difficult to remove in the leaching of eucalyptus chips, Table 3. The decrease of calcium in this case was approximately 52%. Other metal ions were removed to approximately the same extent as in the leaching of birch chips.

Table 2. Content of metals in reference and leached birch wood chips, mg/kg dry wood. The leaching was performed at room temperature (22°C) for 24 hrs at a pH of 2.5.

Table 3. Content of metals in reference and leached eucalyptus wood chips, mg/kg dry wood. Leaching conditions: see Table 2.

The contents of lignin and extractives were virtually unaffected by the acidic leaching stage as shown in Table 4. The gravimetrical yields were 98.7% for birch and 99.3% for eucalyptus. Based on an analysis of the COD in the filtrates from the water impregnation and leaching stages, and assuming that all dissolved substance was carbohydrate material, the calculated yield losses for birch and eucalyptus were 1.2% and 0.2%, respectively. In the case of birch , almost 70% of the COD was dissolved in the water impregnation stage but in the case of eucalyptus, almost all the COD was dissolved in this stage.

Table 4. The gravimetric yield, Klason lignin, acid-soluble lignin and acetone-extractable matter for birch and eucalyptus chips after leaching. All units are based on dry wood prior to any treatment. Leaching conditions: see Table 2.

Effect of leaching and restoration of magnesium and calcium on the rate of delignification and chemical Composition

Untreated (Ref) and acid-leached (A) birch and eucalyptus chips showed, as previously noted, significant differences in the rate of delignification in the subsequent cooking stage, Table 5. In the cooking of leached chips, the rate of delignification was markedly increased in the case of birch but was virtually unaffected in the case of eucalyptus.

Table 5. Kappa number and residual alkali after cooking of reference (Ref.) and acid-leached (A) birch and eucalyptus chips. Leaching conditions; see Table 2. Cooking conditions; EA=20 % (birch) and 17% (eucalyptus), sulphidity=35%, time at 160°C=94 min. (birch) and 49 min. (eucalyptus).

The effects of different variables on the rate of delignification of A-birch chips during kraft cooking was further studied by a series of cooking trials according to Table 6. Prior to cooking, water-impregnated birch chips were leached at room temperature for 24 h at a pH of 2.5 (A series). In the subsequent cooking stage, calcium and magnesium ions were charged together with the white liquor in an amount corresponding to the amount of calcium removed in the leaching stage (A-Ca and A-Mg series). The basis for comparison was the untreated birch chips (Ref. series).

Table 6. Amounts in mmol/kg wood of Ca in reference chips (Ref) and leached chips (A), and amounts of Ca and Mg added in the cooking stage (A-Ca and A-Mg).

Figure 5 shows the kappa number versus the time at maximum cooking temperature for the series according to Table 6. Firstly, it can be concluded that the A-series was delignified at a significantly higher rate than the Ref-series. Secondly, when the calcium content of the A-chips was restored, i.e. the A-Ca series, the rate of delignification was decreased to a rate similar to that of the reference. Finally, if magnesium was instead charged to the cook of the A-chips, A-Mg series, the rate of delignification was the same as for the A-series.

Figure 5. Kappa number versus time in cooking stage at 160°C for Ref. (filled circles), A (hollow squares), A-Ca (hollow triangles) and A-Mg (filled rhombuses) series.

After 94 min. of cooking time, pulp samples of the Ref, A and A-Ca series were analysed with respect to chemical composition, Table 7. The difference in Klason lignin between the Ref -series on the one hand and the A and ACa series on the other was 0.8%. Based on data reported by Li and Gellerstedt (1998), the difference in Klason lignin between the series corresponds to 5.1 kappa number units, which is in good agreement with the observed difference in kappa number of 5.3. No significant differences were observed with respect to carbohydrate yield. Subtracting the contents of Klason lignin and acetone-extractable matter from the total yield results in carbohydrate yields of 47.3%, 47.6% and 47.6% for the Ref, A and A-Ca series respectively. No significant differences were observed in the compositions of monosaccharides.

Table 7. Chemical composition in terms of carbohydrate monomers, lignin and acetone -extractable matter of pulps in % on dry pulp after 94 min. cooking at 160°C. The contents of carbohydrate monomers were weighted so that the sum of carbohydrates, lignin and extractable matter was 100%.

Effect of calcium on pulp brightness and viscosity

Not only the rate of delignification was higher but also the unbleached brightness in the A and A-Mg series than in the Ref and A-Ca series, Figure 6. It is well known that metal ions, especially transition metals, cause an increase in the light absorption of lignin (Falkehag et al . 1966, Peart and Ni 2001). Our results also show that calcium could be an important parameter to consider with respect to the unbleached pulp brightness (cf. Sundin and Hartler 2000b, Sjostrom 1999). In addition, magnesium does not seem to negatively affect the pulp brightness..

Figure 6. ISO-brightness versus kappa number after cooking. Notations according to Figure 5.

Leaching of birch chips prior to cooking also results in a kraft cook with a higher selectivity. The viscosities of all the pulps of the A and A-Mg series were higher than the Ref and A-Ca series, both at a given kappa number, Figure 7, and at a given time, Figure 8.

Figure 7. Viscosity versus unbleached kappa number. Notations according to Figure 5.

Figure 8. Viscosity versus the time at maximum cooking temperature. Notations according to Figure 5.

Calcium in black liquors

The calcium ions interact by some mechanism with the wood components in the fibre wall and with dissolved wood components in the black liquor. It is thus relevant to consider in what state the calcium ions exist during kraft cooking. For example, the method used to restore the calcium content of the wood chips could be important. However, subsidiary studies in our laboratory have shown that there was no difference in effect between calcium restored in a separate stage under neutral conditions prior to cooking and calcium restored by charging it together with the white liquor (unpublished work).

The concentration of calcium in the black liquor is shown in Table 8. The concentrations in the Ref and A-Ca series were approximately the same, ca. 1.7 mmol/l, while the black liquor from the A series contained significantly less calcium, 0.5 mmol/l. In the case of the A-Ca series, the results show that the calcium added with the white liquor has been distributed between the cooking liquor and the wood chips in a manner similar to the case of the Ref series.

Table 8. Concentration of calcium in black liquor, mmol/l, after 94 min. of cooking of birch chips at 160°C.

During the laboratory cooking of birch at a liquor-to-wood ratio of 4:1, 20 – 25 mM carbonate is formed, which is more than 10 times greater than the concentration of calcium. Even so, relatively high concentrations of calcium in a soluble form are found in the black liquor (Hartler and Libert 1973, Lidén 1996). It has also been shown that both the time to reach and the level of the maximum concentration of calcium in black liquor are reduced with increasing concentration of carbonate (Hartler and Libert 1973). It should be emphasized that the white liquor used in the present work was of synthetic quality, i.e. no carbonate was added. Industrially, the concentrations of both carbonate and dissolved wood components are higher than in this laboratory procedure. It is therefore difficult to draw any conclusions regarding the rate of delignification, selectivity and unbleached pulp brightness in a mill situation.

GENERAL DISCUSSION

The observed effect on the rate of delignification could be due to a combination of factors. For example, it is well known that the concentration of divalent cations has a pronounced effect on the leaching of lignin from the fibre wall. A millimolar increase in the calcium or magnesium concentration severely decreases the leaching efficiency (Favis and Goring 1983, Li and MacLeod 1993). Furthermore, both calcium and magnesium have been shown to differ from e.g. sodium with respect to the precipitation of lignin under alkaline conditions (Lindstrom 1980, Sundin and Hartler 2000a). Under conditions resembling those of a kraft cook, comprehensive studies have shown that the precipitation of lignin is enhanced by increasing pOH, temperature and concentration of sodium and calcium ions (Norgren et al. 2000, 2002a). The hydrodynamic diameter of lignin aggregates has been reported to be in the range of 1 500 nm, which is much greater than the diameter of the pores in the fibre wall (Norgren 2002b). Recent work has also suggested that the pore size of the fibre wall could restrict the dissolution of lignin (Andreasson et al. 2003). Even though aggregation of lignin probably occurs in the fibre wall under the conditions applied in the kraft cook, this does not appear to be the governing explanation of the results presented here. If a decrease in the calcium content with a subsequent reduction in the amount of aggregated lignin is the explanation of the increased rate of delignification in the A and A-Mg series, pulping of acid-leached eucalyptus wood chips would also have shown characteristics resembling those of birch, cf. Table 4.

Pulping of acid-leached softwood chips would also have shown a more pronounced effect than has been observed, cf. (Brelid et al. 1998, Gellerstedt and Al-Dajani 1998, Kangas et al. 2002). Nor does the absence of any negative effect due to the addition of magnesium provide any explanation regarding the aggregation of lignin as a restricting factor. Structural features of birch and eucalyptus lignin could influence the tendency for aggregation, but no such systematic studies have been found in the literature.

Delignification in the initial and bulk phases of the kraft cook has generally been ascribed to cleavage of phenolic α- and β-arylethers and non-phenolic β-arylethers (Gierer and Norén 1980). In this respect, the chemical model of the delignification in the kraft cook does not explain any effects of e.g. calcium. If, however, the hemicellulose and wood morphology is considered, the effect of calcium concentration on delignification may at least be speculated upon.

During the pulping of birch, hemicelluloses have been found to have an impact on the rate of delignification. For example, birch pulps produced in a flow-through cook were found to have a higher content of leachable lignin compared to pulps produced according to conventional autoclave methods (Hortling et al. 1994). It was assumed that this was due to a higher content of xylan in the conventionally produced than in the flow-through produced pulp. Xylanase treatment of the pulps prior to leaching also improved the leachability with respect to lignin, further indicating that dissolution of lignin is restricted by the xylan (Hortling et al. 1994). Earlier studies have also indicated that removal of hemicellulose with cold soda extraction prior to chlorite pulping increases the rate of delignification of birch wood (Kerr and Goring 1975).

Over the years, a number of studies have discussed the interactions between lignin and carbohydrates and their possible effects on the dissolution of lignin. Recently, Lawoko et al. reported a possible explanation of the difficulties in removing the residual lignin in kraft cooking (Lawoko et al. 2004). Basically, it was suggested that lignin bound to the hemicellulose is interlocked between the hemicellulose and the crystalline cellulose and thereby restrict the dissolution of lignin from the fibre wall. Although the exact organisation of hemicelluloses in the wood fibre is not completely known, it is highly probable that they are organized parallel to the cellulose fibrils (Salmén 2003). During pulping of birch, it has furthermore been shown that xylan may crystallize onto the cellulose fibres (Marchessault 1967). The crystallisation of xylan could possibly further reduce the dissolution of lignin by interlocking. The role of calcium in this respect could possibly be a bridging between the charged groups in the xylan polymer, thus supporting sorption and/or crystallisation of xylan (and lignin bound to the xylan) and thereby increasing the interlocking of the lignin. Calcium is known to be able to bind to acidic groups in pectins and could, by analogy with the proposed "egg-box" model of pectic polymers (Grant 1973), act in an equivalent manner between the charged groups in the xylan polymers.

This reasoning does not however seem to be in compliance with the results obtained in the case of pulping of leached eucalyptus chips. However, the structure of xylan in eucalyptus differs from that in birch. Unlike birch xylan, eucalyptus xylan has been reported to have a branched structure; 30% of the MeGlcA sidegroups are substituted with rhamnoarabinogalactan at O-2 of the MeGlcA group (Evtuguin et al. 2004, Pinto et al. 2003). There are also indications that there are differences between eucalyptus and birch with respect to lignin and lignin-carbohydrate bonds. Isolated lignin from eucalyptus kraft pulp was reported to contain a higher content of glucose than that from birch, implying that lignin -cellulose bonds are more frequent in eucalyptus than in birch (Capanema et al. 2004). The larger proportion of lignin bound to xylan in birch than in eucalyptus combined with a possible bridging of xylan polymers by calcium could be one of the factors explaining the absence of any effects of leaching in the case of eucalyptus, although the effect was significant in the case of birch.

CONCLUSIONS

From the trials on acidic leaching of birch and eucalyptus chips, it can be concluded that:

• Leaching of calcium is more difficult for eucalyptus chips than compared to birch chips.

• Potassium, magnesium and manganese are leached equally for birch and eucalyptus chips.

• A substantial removal of chloride is achieved during leaching of eucalyptus chips.

• Acidic leaching significantly improves the unbleached brightness for birch and to some extent for unbleached eucalyptus pulp.

• In the case of pulping of birch, acidic leaching improves the rate of delignification and cooking selectivity. No positive effect is observed in the case of pulping of eucalyptus.

ACKNOWLEDGEMENTS

Lars Norberg, STFI-Packforsk AB is gratefully acknowledged for his skilful laboratory work. This work is a part of a R&D programme titled "Future resource adapted pulp mill – FRAM". We are very grateful to STEM (the Swedish Energy Agency), MISTRA (the Foundation for Strategic Environmental Research), Ångpanneföreningens Foundation for Research and Development, Södra Cell, Stora Enso, Fortum Värme, Sydkraft , Borregaard LignoTech, Orelis, EPCON, Holmen Paper, Anox, Bäckhammars bruk and Aga Linde for their support and their commitment to the programme.

REFERENCES

Andreasson, B., Forsstrom, J., Wagberg, L. – Cellulose 10:111(2003)

Axegard, P. – Ecocyclic Pulp Mill – "KAM", KAM report A100, p. 83-89, available from STFI-Packforsk AB (2003)

Brelid, H., Friberg, T., Simonson, R. - Nord. Pulp Paper Res. J. 13(1):50(1998)

Brelid, H. - 7th International Conference on New Available Technologies, Stockholm, Sweden, p.17(2002)

Capanema, E.A., Balakshin, M.Y., Chen, C-L. – Holzforschung 58(5):464(2004)

Evtuguin, D.V., Domingues, M.R., Amado, F.M.L., Neto, C. – 8th Europ. Worksh. on Lignocellulosics and Pulp, Riga, Latvia, 22-25 aug.:45(2004)

Falkehag, I., Marton, J., Adler, E. – Adv. Chem. Ser. 59:75(1966)

Favis, B.D., Goring, D.A.I. – J. Wood Chem. Tech. 3(1):9(1983)

Gellerstedt, G., Al-Dajani, W.W. - 5th European Workshop on Lignocellulosics and Pulp, Aveiro, Portugal, p.547(1998)

Gierer, J., Norén, I. – Holzforschung 34:197(1980)

Grant, G.T., Morris, E.R., Rees, D.A. – FEBS letters 32(1):195(1973)

Hartler, N., Libert, J. – Svensk Papperstidning 76(12):454(1973)

Hortling, B., Korhonen, M., Buchert, J., Sundquist, J., Viikari, L. – Holzforschung 48(5):441(1994)

Kangas, H., Robertsén, L., Vuorinen, T. – P. ja Puu 84(7):473(2002)

Kerr, A.J., Goring, D.A.I – Can. J. Chem. 53:952(1975)

Lachenal, D. - 347-346 In Pulp Bleaching Dence C.W., Reeve, D.W. Eds.: TAPPI PRESS, Atlanta, GA (1996)

Lawoko, M., Berggren, R., Berthold, F., Henriksson, G., Gellerstedt, G. – Holzforschung 58(6):603(2004)

Li, J., Gellerstedt, G. – Nord. Pulp Pap. Res. J. 13(2):153(1998)

Li, J., MacLeod, J.M. – J. Pulp Pap. Sci. 19(2):J85(1993) Lidén, J., Lindgren, P., Soderberg, C . – 5th Int. Conf. on New Avail. Tech., Stockholm, Sweden, Part I, p. 498(1996)

Lindgren, K., Berglin, N., Brelid, H. - International Pulp Bleaching Conference Portland, OR, USA .315(2002)

Lindstrom, T. – Colloid & Polymer Sci. 258:168(1980)

Marchessault, R.H., Settineri, W., Winter, W. – Tappi J. 50(2):55(1967)

Norgren, M., Edlund, H., Wagberg, L., Lindstrom, B., Annergren, G. – Colloids and Surfaces 194:85(2000)

Norgren, M., Edlund, H., Wagberg, L., Annergren, G. – Nord. Pulp Pap. Res. J. 17(4):370(2002a)

Norgren, M., Edlund, H., Wagberg, L. – Langmuir 18:2859(2002b)

Peart, C., Ni, Y. – J. Wood Chem. Tech. 21(2):113(2001)

Pinto, P., Evtuguin, D.V., Neto, C.P. – 12th Int. Symp. Wood and Pulp. Chem., June 9-12, Madison, USA, Vol. II:87(2003)

Salmén, L. – Comptes Rendus Biologies 327(9- 10):873(2004)

Saltberg, A., Brelid, H. - The 13th International Symposium on Wood, Fibre and Pulping Chemistry, Auckland, New Zealand (2005)

Sundin, J., Hartler, N. – Nord. Pulp Pap. Res. J. 15(4):306(2000a)

Sundin, J., Hartler, N. – Nord. Pulp Pap. Res. J. 15(4):313(2000b)

Sjostrom, K. – 38-39 Some Aspects on TCF-bleachability of Softwood Alkaline Pulps, Doctoral Dissertation, KTH, Stockholm (1999)

Ulmgren P. - Nord. Pulp Pap. Res. J. 12(1):32(1997)

Ulmgren, P., Radestrom, R. – TAPPI Minimum Effluent Mills Symposium, San Francisco, CA p. 51(1997)

Westervelt, W., Frederick, J.W., Malcom, E.W. – Tappi J. 65(5):179(1982)

Werner, J. A., Ragauskas, A.J, Jiang, J.E. – Tappi Pulping Conf., Montreal, Quebec, Canada, Proc. Vol III:1145(1998)

Authors contact details:
Fredrik Lundqvist¹, Harald Brelid², Anna Saltberg² , Göran Gellerstedt³, Per Tomani¹

1 STFI-Packforsk AB, Division of Fibre, Pulp and Energy, Stockholm, Sweden

2 Chalmers University of Technology, Forest Products and Chemical Engineering, Gothenburg, Sweden

3 Royal Institute of Technology, KTH, Divison of Wood Chemistry and Pulp Technology, Stockholm, Sweden