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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
In the untreated pulp must be ensured in order to avoid a change in the supramolecular
Structure while attaining a sufficiently low xylan content to meet the required
Specifications for high-purity pulps (see Section 11.3, Tab. 11.7, Pulp properties).
The relationship between initial pulp purity (R18) and final xylan content
Cold Caustic Extraction
0 20 40 60 80 100
unbleached HW-S (E/O) treated HW-S O-Z treated E-PHK
Xylan content [%]
NaOH concentration [g/l]
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Lattice planes is widened from the original 0.61 nm to more than 1.2 nm due | | | Fig. 8.5 Purification of hardwood sulfite pulps |