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BLACK LIQUOR COMPONENTS AS POTENTIAL RAW MATERIALSKlaus Niemel, Tarja Tamminen, Taina Ohra-aho

Presented at 14th ISWFPC - Durban, South Africa

Key words: Kraft pulping, pine, black liquor, composition, lignin, hemicellulose, by-product,hydroxy acid, tall oil

ABSTRACTPine wood chips were cooked under different kraft pulping conditions which represented threedifferent delignification rates: 1) typical, 2), lower rate (lower sulphidity), and 3) higher rate(higher temperature). The effect of the cooking conditions on black liquor composition wasmonitored during the course of pulping. Representative hemicellulose and lignin fractions werealso isolated for deeper characterization.

The cooking conditions had varying effects on the amounts and structure of all main blackliquor components, lignin, hemicelluloses and their degradation products, as well as extractives.The highest amount of carbohydrates was found in black liquor after the high-delignificationrate cooking; in the bulk delignification stage their amount corresponded to 4.5% of wood;xylan was the dominating polysaccharide. In the initial cooking stages, more substantial

amounts of arabinan and galactan were present in the black liquors. The cooking conditionsalso affected the yield of carbohydrates that could be isolated from black liquors with dioxaneand acetic acid the yield varied from 10 to 93%. The isolated hemicelluloses contained somelignin, but the presence of non-lignin type aromatic constituents was also evident: pyrolysisGC/MS indicated that 1030% of the phenolic degradation products from the hemicellulosefractions are of non-guaiacyl type.

The cooking conditions had certain effects on the molar mass and sulphur and carbohydratecontents of kraft lignins. Depending on the conditions, the dominating carbohydrate was eithergalactan or xylan. The sulphur content varied between 0.9 and 2.7, being lowest after the lowsulphidity cooking. The formation of acetic acid, formic acid, and various hydroxy acids duringcooking was also monitored. Their total amount, more than 10% of wood, was not dramaticallyaffected by different cooking conditions.

1. INTRODUCTION

Currently, the main by-products from kraft pulping include tall oil1,2 and turpentine3,4, and theannual global production figures for these are 1.5 million and 0.10.2 million tonnes,respectively. In addition to these by-products, small amounts (less than 0.1 million tonnes) of

kraft lignin are also isolated from black liquors for chemical applications5. The carbohydrate-

derived fractions6, polysaccharides and aliphatic carboxylic acids, are not currently isolatedfrom black liquor and used as by-products. Their exploitation has been reduced by scatteredinformation on their chemical nature or composition, isolation and purification problems, as wellas limited markets.

In this work, we have now isolated hemicelluloses and lignin from pine kraft black liquors(during the course of pulping) for further characterization, and we have also analyzed the mainaliphatic carboxylic acids. For deeper information on their formation and factors controllingthem, our study was based on three slightly different cooks: normal (reference) cook and othercooks with significantly higher and lower delignification rates. A selection of the main resultsare now reported and briefly discussed, mainly from an analytical point of view.

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Table 3.1. Relative monosaccharide composition (%) of hemicelluloses in the black liquors.*Hemicelluloses isolated for further characterization.

The hemicelluloses were isolated with dioxane and acetic acid from five black liquors (Table3.1), with widely varying yields (calculated from their total amounts). The yield was 93% forHA-H350 and R-H1000, 59% for R-H30, 34% for R-H2500, but only 10% for LS-H3000. Theirmonosaccharide compositions were analyzed by pyrolysis GC/MS (all samples) and acidhydrolysis HPLC method (R-H1000 and HA-H350). It thus became evident that thecomposition of the total carbohydrates and those recovered by organic solvents may differ alot, particularly if the isolation yields remain low. Typically, the isolated hemicelluloses

contained somewhat more xylose and glucose, and less arabinose and galactose, than the bulkhemicelluloses present in the black liquors.

The hemicellulose preparations isolated with dioxane and acetic acid are known to contain circa

5% of lignin as the main impurity9,10. The lignin impurities were characterized by pyrolysisGC/MS, and compared with the corresponding kraft lignins. The comparison revealed a numberof interesting features. It also indicated the presence of aromatic structures that are notcharacteristic of lignin.

Relative distribution of the guaiacyl-type degradation products from the isolated hemi-celluloses is compared with the products from the corresponding kraft lignin in Fig. 3.1. Themost striking differences include more pronounced formation of guaiacol from the hemicellulose

fractions, whereas the amount of 4-methylguaiacol is clearly decreased. It is likely that thesechanges are associated with the presence of lignin-carbohydrate bonds, although more detailedstructural speculations are not currently possible.

In addition to the guaiacyl-type compounds, other phenolic compounds were also liberatedfrom the hemicellulose and lignin fractions during pyrolysis (and identified by GC/MS). Theiryield was 35% (of all aromatic products) from the lignin fractions, but up to 1030% from thehemicellulose fractions. It also shows that their relative amount in the hemicellulose fractionincreases during the course of pulping. This was clearly revealed by the reference cook series:the share of the other aromatic degradation products from hemicelluloses was 10% at H

30,

13% at H1000, and 30% at H2500. In the bulk delignification stages of the high alkalinity andlow sulphidity cooks, the corresponding figures were 15 and 10%, respectively.

The nature of the non-guaiacyl aromatic pyrolysis products appears in Fig. 3.2, showing distinctdifferences between the hemicellulose and lignin fractions. Some products (hydroquinone and 4-hydroxybenzaldehyde) were liberated only from the hemicellulose fractions, whereas moresubstantial amounts of catechol and 4-methylphenol were derived from the lignin fractions thanfrom the hemicelluloses.

It is reasonable to assume that certain non-guaiacyl-type pyrolysis products (particularlyhydroquinone) derived from hemicelluloses originate from specific aromatic structures, formedinto the polysaccharide chains by aromatization reactions during cooking.

R-H1500 26.7 26.7 2.9 43.7 +

R-H2500* 28.0 36.9 4.3 30.8 +

HA-H30 32.3 47.4 4.1 16.2 +

HA-H350* 15.7 10.5 2.2 67.0 4.6

HA-H750 14.9 13.9 2.6 64.5 4.2

LS-H30 30.9 49.3 5.0 14.8 +

LS-H3000* 26.4 35.0 3.5 35.0 +

LS-H6000 27.9 48.0 6.1 17.9 +




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Fig. 3.1. Distribution of the guaiacyl-type degradation products derived from the isolated hemicellulose and lignin

fractions, as analyzed by pyrolysis GC/MS.

Fig. 3.2. Distribution of non-guaiacyl-type degradation products from the analytical pyrolysis of the isolated

hemicellulose and lignin fractions.

3.2 Kraft lignins

All the black liquors were analyzed for the lignin contents (Appendix 6.1) and characterized forlignin phenolic groups and molar mass distributions. The main focus is now given, however, onthe selected properties of lignins isolated from five black liquors for further characterization(Table 3.2).

All the isolated kraft lignins contained some carbohydrates, varying from 1 to 9%. The highestamount of them were present in lignin from the high alkalinity cook, in agreement with thetotal carbohydrate contents of the black liquors (Appendix 6.1).

Table 3.2. Selected properties of isolated pine kraft lignins.*Percent of total phenolic OH groups.

Xylose and galactose were the dominating monosaccharide units in the lignin-boundcarbohydrates (Fig. 3.3). The amount of galactose was especially high at the end of thereference cook, and in the bulk delignification stage of the low sulphidity cook.

Property R-H30 R-H1000 R-H2500 HA-H350 LS-H3000

Carbohydrates, % 1.0 4.0 2.5 9.2 2.7

Extractives, % 13.5 2.7 2.2 2.4 3.8

Sulfur, % 2.5 1.7 1.7 1.8 0.9

Phenolic OH groups, mmol/g 2.3 2.8 2.9 2.9 2.8

Conjugated phenolic groups, %* 17 13 11 14 10

Molar mass (Mw), mol/g 4460 6580 5980 9860 7070




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Fig. 3.3. Relative monosaccharide composition (%) of lignin-bound carbohydrates.

The amount of extractives in the isolated lignins varied from circa 2 to 4%, apart from thelignin fraction isolated at an early stage of cooking (R-H30, 13.5% of extractives). The GCanalysis indicated that in each case fatty and resin acids were the main constituents, althoughsome sterols and other compounds were also present. An important impurity of the isolatedkraft lignins is organically bound sulphur. Its amount was expectedly the lowest (only 0.9%)after the low sulphidity cooking. The results from the pyrolysis GC/MS studies were alreadydiscussed in Section 3.1.

The amounts of the phenolic hydroxyl groups were little affected by the cooking conditions (orcooking stages). The molar mass of kraft lignin was clearly highest after the high alkalinitycooking.

3.3 Aliphatic carboxylic acidsThe total amount of low-molecular-weight carboxylic acids (after the bulk delignification stages)was more than 20 g/L (Table 3.3), corresponding to c. 10% of wood. However, it appears thatthe amount of the hydroxy acids was now somewhat lower than could perhaps be expected onthe basis of previous studies (e.g. ref. 18). In any case, these types of compounds are formedin substantial amounts during cooking, as a result of the degradation of polysaccharides(mainly hemicelluloses).

Most of acetic acid was formed (from the acetyl groups of glucomannan) during the initial stageof cooking, whereas the formation of the other carboxylic acids proceeded more or lessconstantly during the entire cooking.

Table 3.3. Concentrations (g/L) of low-molecular-weight carboxylic acids in the black liquors.

The main hydroxy monocarboxylic acids included glycolic, lactic, 2-hydroxybutanoic, 2,5-

dihydroxypentanoic, xyloisosaccharinic, and isomeric glucoisosaccharinic acids, although a largenumber of minor carboxylic acids were also identified. Their relative amounts were nowgenerally in a good agreement with the previous studies6,18 and are not listed in more detail.In addition, small amounts of aromatic hydroxy carboxylic acids, -guaiacyl-2-hydroxyalkanoicacids19, could also be identified. Their formation requires condensation reactions betweenlignin-derived and carbohydrate-derived fragmentation intermediates.

As a whole, it appears that the different cooking conditions did not have dramatic effects on theformation of various low-molecular weight carboxylic acids. However, the results suggest thatthe relatively slow low sulphidity cooking has slightly favoured their formation.

Extractives (crude tall oil) were also isolated from the black liquors after the bulk delignification

stages, and analyzed for individual compounds. The only striking feature worth mentioning isthat the high alkalinity cook (at the temperature of 180 C) had resulted in some losses ofunsaturated fatty acids, especially linoleic acid.

4. CONCLUSIONSThe present results demonstrate that different pulping conditions can result in various changes

Carboxylic acid R-H1000 LS-H3000 HA-H350

Formic acid 6.3 8.0 6.3

Acetic acid 4.2 3.7 3.5

Hydroxy mono- and dicarboxylic acids 9.8 11.0 10.8

Total 20.3 22.7 20.6




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in the composition and structure of the main black liquor compounds, although such differencesare seldom very distinct. This type of information can be used to search for pulping conditionssuitable for the production and isolation of potential raw materials (by-products) for variousapplications. Naturally, the pulp quality should not be compromised.

It became evident that the total amount of carbohydrates, their relative composition, and theirrecovery with organic solvents (at least with dioxane acetic acid) can depend, to a largeextent, on the cooking conditions. Typically, however, hardwood kraft black liquors areexpected to contain more substantial amounts of hemicelluloses (especially xylan) than

softwood black liquors, providing other opportunities for their recovery. It was also found thatthe extent of carbohydrate bonding with kraft lignin, to hamper their isolation, may be affectedby the cooking conditions.

Several important kraft lignin properties (such as sulphur content and molar mass distribution)may also depend on the cooking conditions, although certain other properties (such as theamount of reactive phenolic hydroxyl groups) seem to be more constant.

The aliphatic carboxylic acids form an interesting but little studied fraction of potential by-products. Their separation has attracted some interest in the past20,21 but has not yet beenrealized in industrial scale. The main hydroxy carboxylic acids include products (such as lacticand glycolic acids) with well-established applications, whereas the properties and uses of some

others (especially isosaccharinic acids) would still require further studies.

5. LITERATURE CITED1.Lofthouse R. The European pine chemicals industry, 1987-1997. For. Chem. Rev. 108(1998):6, 1114.2.Ragaskaukas A.J., Nagy M., Kim D.H., Eckert C.A., Hallet J.P., Liotta C.L. From wood to fuels:integrating biofuels and pulp production. Ind. Biotechnol. 2 (2006):1, 5565.3.Sainte-Cluque P. Global overview of crude sulphate turpentine. For. Chem. Rev. 109(1999):1, 810.4.Hinson J.M. Worldwide turpentine outlook 2002: optimism or concern? For. Chem. Rev. 112(2002):6, 1215.5.Pye E.K. Industrial lignin production and applications. In: Biorefineries industrial processand products (Eds. B. Kamm, P.R. Gruber, M. Kamm), Wiley-VCH, Weinheim, Germany, Vol. 2,pp. 165200.6.Niemel K., Aln R. Characterization of pulping liquors. In: Analytical methods in woodchemistry, pulping, and papermaking (Eds. E. Sjstrm, R. Aln), Springer, Berlin, 1999, pp.193231.7.Hausalo, T. Analysis of wood and pulp carbohydrates by anion exchange chromatographywith pulsed amperometric detection. 8th International Symposium on Wood and PulpingChemistry, Helsinki, Finland, June 69, 1995, Vol. III, 131136.8.Tamminen T., Hortling B. Isolation and characterization of residual lignin. In: Progress inlignocellulosics characterization (Ed. D. Argyropoulos), TAPPI Press, Atlanta, USA, 1999, pp. 142.9.Engstrm N., Vikkula A., Teleman A., Vuorinen T. Structure of hemicelluloses in pine kraftcooking liquors. 8th International Symposium on Wood and Pulping Chemistry, Helsinki,

Finland, June 69, 1995, Vol. III, pp. 195200.10.Vikkula A. Hemicelluloses in kraft cooking liquors. Licentiate Thesis, Helsinki University ofTechnology, Finland, 81 p.11.Tamminen T., Vuorinen T., Tenkanen M., Hortling B. Analysis of lignin and lignin-carbohydrate complexes isolated from black liquor. 8th International Symposium on Wood andPulping Chemistry, Helsinki, Finland, June 69, 1995, Vol. II, pp. 297302.12.Tamminen T., Ohra-aho T., Hortling B., Tenkanen M. Residual lignin in hydrogen peroxide-bleached softwood pulps. 12th International Symposium on Wood and Pulping Chemistry,Madison, WI, USA, June 912, 2003, Vol. I, pp. 6972.13.Hortling B., Turunen E., Kokkonen P. Molar mass and size distribution of lignins. In:Handbook of size exclusion chromatography and related techniques (Ed. C.-s. Wu), 2nd ed.,Marcel Dekker Inc., 2003, pp. 355384.

14.Aln R., Niemel K., Sjstrm E. Gas-liquid chromatographic separation of hydroxymonocarboxylic acids and dicarboxylic acids on a fused-silica capillary column. J. Chromatogr.301 (1984), 273276.15.Tamminen T., Ranua M., Dufour B., Kokkonen R., Kauliomki S. Filtrate analysis as tool tofollow peroxide bleaching performance. Papel 68 (2007):2, 8291.16.Saltsman W., Kuiken K.A. Estimation of tall oil in sulphate black liquor. Tappi 59 (1959),




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