Groundwood Acid Sulfite Bisulfite Kraft

Tear Index [mNm2/g]

Unbleached Spruce Pulps

Tensile Index [Nm/g]

Tensile Index

Tear Index

Fig. 4.1 Paper properties of unbleached spruce pulps at 45°

SR, showing the effect of different pulp processes (according

to Annergren and Rydholm [1]).

Handbook of Pulp. Edited by Herbert Sixta

Copyright © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

ISBN: 3-527-30999-3

©2006 WILEY-VCHVerlag GmbH&Co.

Handbook of Pulp

Edited by Herbert Sixta

On a global scale, pulps are predominantly produced by chemical pulping processes.

In 2000, the chemical pulps accounted for more than 77% of all woodbased

fiber material worldwide (see Tab. 2 in Chapter 1, Introduction) [2]. In

chemical pulping, lignin is degraded and dissolved through chemical reactions at

elevated temperatures (130–170 °C). The fibers can be separated without further

mechanical defibration only after about 90% of the lignin has been removed. Unfortunately,

delignification is not a selective process. Parallel to the lignin removal,

significant parts of the hemicelluloses and some cellulose are also degraded. The

total fiber yield ranges from 45 to 55% (at a given extent of delignification of about

90%), depending on the wood source and the pulping process applied. Continuing

cooking beyond a certain extent of delignification inevitably results in disproportionately

large yield losses due to preferred carbohydrate degradation. Hence,

the chemical reactions must be stopped at a point when the lignin content is low

enough for fiber separation, and where acceptable yield can be still attained. In a

complete fiber line, further delignification is achieved by bleaching processes

downstream of the digester.

The main commercial chemical pulping techniques comprise the sulfate or

kraft, the acid sulfite, and the soda processes. The dissolution of wood components

during pulping is characteristic for each pulping process which, at a given

residual lignin content, is reflected in the carbohydrate yield and composition.

Cellulose is largely preserved in sulfite pulps, whereas xylan is most stabilized in

kraft pulps. The high resistance of xylan towards alkali is the main reason for the

Tab. 4.1 Yields of main pulp components after acid Mg sulfite [4,5]and kraft

pulping [4,6]of beech and spruce wood.