Wood Species

The wood raw material has a decisive influence on the processability and final

pulp quality. The heterogeneous nature of the wood structure and the chemical

and physical differences between and within wood species are reflected in all

pulping processes. Acid sulfite pulping technology is known to be the most sensitive

process to the properties of the wood raw material, and many softwood species

– and also some hardwoods – are considered to be less suited to acid sulfite

pulping. The deficiencies involved with acid sulfite pulping of, for example pines

or larches, are high screenings, a high residual lignin content, and thus a low

screened yield. The poor pulping results are connected with either the presence of

certain extractives in the wood, especially in the heartwood, or limited impregnation

due to a dense wood structure. It has been found that phenolic resins – such

as pinosylvin in pine heartwood or taxifolin in Douglas fir – react with lignin in

acid sulfite liquors to form a condensation product that prevents delignification

[24–27]. Moreover, taxifolin tends to reduce hydrogen sulfite ions to thiosulfate

ions, thereby decreasing the stability of the sulfite cooking liquor [28].

Subsequently, much effort has been undertaken to modify the sulfite cooking

process to overcome the problems encountered with the use of resinous wood. It

was found that pretreatment of the wood with a sulfite cooking liquor of moderate

acidity, as with a pure hydrogen sulfite solution, and low temperature will favor

sulfonation of the reactive groups of the lignin over condensation reactions with

the phenolic resins. Another prerequisite of selective sulfonation comprises an

efficient pressure impregnation. This initial sulfonation stage is then followed by

a conventional acid sulfite cook at temperatures and H-factors selected according

to the grade desired. Hence, two-stage sulfite pulping with a preceding sulfonation

stage at low acidity or even neutral pH conditions makes possible the use of

pines and larches.

4.3 Sulfite Chemical Pulping 449

In the case of conventional and more simple one-stage acid sulfite pulping processes,

only a limited amount of wood species can be used. Most appropriate are

hardwoods with low resin contents and high density such as beech wood (Fagus

sylvatica) and certain eucalyptus species (E. globulus, E. saligna, E.urograndis, etc.).

Dense hardwoods with a low lignin content are favorable because of the high specific

pulp yield related to the volume of the digester. Replacing spruce with, for

example beech wood, results in an almost 50% higher yield at a given digester

volume (280 kg o.d. beech versus 190 kg o.d. spruce per m3 digester, respectively).

In the following section, four representative hardwoods – beech (Fagus sylvatica),

aspen (Populus tremulus), eucalypt (E. globulus) and birch (Betula pendula)

and one softwood species, spruce (Picea abies) – which together constitute the

most important wood raw material for acid sulfite pulping, are evaluated comparatively

with respect to their processability and unbleached pulp quality.

The chemical composition of the wood determines very important parameters

such as pulp yield and pulp properties. The purity of dissolving pulps is governed

by the content of noncellulosic carbohydrates and other impurities such as resins

or inorganic components of the wood raw material used.

Chemical analysis of the hardwood samples reveals high glucan and low xylan

contents for aspen and eucalypt. The low glucan and high xylan contents of birch

and beech, however, indicate low cellulose yield at high residual xylan contents in

the resulting dissolving pulps. Among the hardwood species, aspens shows a surprisingly

high mannose content. Spruce, as the only representative of softwoods

in this comparison, contains a relatively high glucan content and, together with

eucalypt, the lowest amount of hemicelluloses among the species investigated.

The hardwoods are characterized by a low and rather constant Klason lignin content

ranging from 18.9% to 21.0%, and by a high acid-soluble lignin content, with

the highest value by far determined for eucalypt. The low lignin content of the

hardwoods indicates a more efficient delignification as compared to spruce at given

cooking conditions.

The highest DCM-extractives contents were measured for birch and aspen, but

this may relate to a pitch problem during further processing of the corresponding

unbleached pulps.

Based on the chemical analysis of the main wood components, the highest cellulose

yields and cellulose purities can be expected for aspen and eucalypt. The

results of the chemical analysis of selected wood chips are detailed in Tab. 4.61

(see also Tab. 4.49).

The introduced five wood species were subjected to one-stage acid sulfite pulping

according to the procedure described in Section 4.3.5.1 (see also Fig. 4.157).

The cooking conditions applied namely the impregnation procedure, the cooking

acid composition, the liquor-to-wood ratios and the maximum cooking temperatures

were identical for beech, aspen, and eucalypt and comparable for spruce and

birch (Tab. 4.62). The slightly different cooking acid compositions in the case of

birch and spruce can be classified as rather insignificant with respect to their ion

product, [H+]·[HSO3

– ], which determines the rate of delignification.

450 4 Chemical Pulping Processes

Tab. 4.61 Chemical composition of beech, aspen, eucalyptus,

birch and spruce as used for the cooking experiments

(according to [14]).

Beech

Дата добавления: 2015-10-21