Investigated using solid-state cp-mas 13c-nmr spectroscopy (Fig. 8.4).

0 100 200 300 400

0

20

40

60

80

100

Eucalyptus-PHK: Cellulose I Na-cellulose I Na-cellulose II

Beech-Sulfite: Cellulose I

Proportion [%]

NaOH concentration [g/l]

Fig. 8.4 Lattice transition from cellulose I to Na-cellulose I

and Na-cellulose II of beech sulfite and eucalyptus-PHK pulps

depending on NaOH concentration. Data were recorded

using solid-state CP-MAS 13C-NMR spectroscopy (according

to [22]).

Over the range of NaOH concentration from about 160 g L–1 to 270 g L–1, the

structure of Na-cellulose I prevails, while beyond this concentration level a further

lattice conversion to Na-cellulose II arises. The NMR-spectrum of this lattice type

Indicates cleavage of the intramolecular hydrogen bond between o-3-h and o-5′,

and thus the coordination of an additional Na+ ion to O-3 [23]. A series of comprehensive

reports provides further information on the changes in supramolecular

structure that occur during the treatment of cellulose with aqueous solutions

[20,22,24–26].

941

8 Pulp Purification

8.3

Cold Caustic Extraction

The extent of purification, measured in terms of R18 and R10 values and residual

hemicellulose content (xylan in case of hardwood pulp), depends primarily on the

NaOH concentration and the temperature (see Section 8.2). Additionally, the reaction

time, the position of the cold caustic extraction (CCE) within the sequence,

and the presence of dissolved hemicelluloses may have an influence on the efficiency

of purification. In industrial CCE treatment, emphasis is placed on efficient

washing. The pulp entering the CCE stage must be thoroughly washed and

dewatered to a high consistency (>35%) in order to avoid dilution of the added

caustic solution through the pulp slurry. The conditions of CCE include the

homogeneous distribution of pulp in 5–10% NaOH for at least 10 min at temperatures

between 25 and 45 °C in a downflow, unpressurized tower. Due to the rapid

Interaction between alkali and cellulose, a separate retention tower is not really

needed (in industrial praxis, a tower would perfectly act as a surge tank). Removal

of the lye from the highly swollen pulp is rather difficult, and requires efficient

post-CCE washing in a series of more than three washers [27]. Special attention

must be paid to the washing concept in order to avoid reprecipitation of dissolved

polymeric hemicelluloses (beta-cellulose) during the course of washing.

The most important parameters influencing the degree of purification are presented

In the following section.

8.3.1

NaOHC oncentration

As anticipated, the hemicellulose content in the pulp, determined as xylan,

decreases linearly with increasing NaOH concentration in the aqueous phase of

the pulp suspension, up to a value of about 100 g L–1 (Fig. 8.5).

Parallel to the decrease in residual xylan content, an increase in R18 content

can be observed. The course of R18 content during CCE treatment as a function

of NaOH concentration is illustrated graphically in Fig. 8.6.

The purification efficiency of both sulfite and PHK pulps is quite comparable,

provided that the initial xylan contents are at the same level. The xylan content of

the unbleached sulfite pulp was reduced by a mild hot caustic extraction followed

by oxygen delignification without interstage washing ((E/O)-stage). Purification

during CCE proceeds for both pulps to levels close to 1% xylan (or slightly below),

even at NaOH concentrations significantly lower than 100 g L–1, which prevents

the conversion of significant parts to Na-cellulose I (see Fig. 8.4). A subsequent

change of the crystalline lattice to the cellulose II-type alters the fiber structure

and thus deteriorates pulp reactivity towards acetylation [29]. A xylan content of about