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3% 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

942

8.3 Cold Caustic Extraction

0 20 40 60 80 100

0

1

2

3

4

5

unbleached HW-S (E/O) treated HW-S O-Z treated E-PHK

Xylan content [%]

NaOH concentration [g/l]

Fig. 8.5 Purification of hardwood sulfite pulps

(HW-S) and eucalyptus prehydrolysis-kraft

pulp (E-PHK) with cold aqueous NaOH solution

of varying strength [28]. HW-S:

unbleached, kappa number 6; (E/O) pretreated:

kappa 1.6. E-PHK: OZ pretreated,

kappa 0.6. CCE-treatment: 10%consistency ,

30 °C, 30 min.

0 20 40 60 80 100

90.0

96

97

98

99

O-Z treated E-PHK

R18 content [%]

NaOH concentration [g/l]

Fig. 8.6 Purification of eucalyptus prehydrolysis-kraft pulp (EPHK)

with cold aqueous NaOH solution of varying strength

[28]. E-PHK: OZ pretreated, kappa number 0.6. CCE-treatment:

10%consistency , 30 °C, 30 min.

943

8 Pulp Purification

through alkaline treatment, depending on NaOH concentration, is further illustrated

in Fig. 8.7. The different levels of R18 content after cooking and subsequent

oxygen delignification (O) of the eucalyptus PHK pulps have been adjusted by prehydrolysis

intensity (P-factor). Even though xylan removal efficiency increases

with increasing initial hemicellulose content, the initial purity must exceed a certain

level in order to achieve a sufficiently high purity without approaching a

change in the supramolecular structure.

0 20 40 60 80 100

0.0

1

2

3

4

5

Initial R18 content:

96.6% 97.2% 97.6%

Xylan content [%]

NaOH concentration [g/l]

Fig. 8.7 Purification of eucalyptus prehydrolysis-

kraft pulps (E-PHK) of three different initial

purity levels with cold aqueous NaOH solution

of varying strength [28]. O-pretreated E-PHK

pulps: (a) R18 = 96.6%, kappa number 3.4; (b)

R18 = 97.4%, kappa number 2.5; (c)

R18 = 97.6%, kappa number = 2.2. CCE-treatment:

10%consistency , 30 °C, 30 min.

8.3.2

Time and Temperature

The influence of temperature on the performance of caustic extraction has been

discussed in detail in Section 8.2. It is well established that lower temperatures

cause a high degree of swelling, and this enhances the solubility of hemicelluloses.

The effect of temperature and retention time in the range 30–50 °C and 10–

60 min, respectively, while keeping the NaOH concentration constant at 70 g L–1,

is illustrated in Fig. 8.8.

It can be seen that extraction time has no influence on the purification efficiency

in the range investigated. The retention time during alkalization is not a

critical parameter because swelling takes place almost instantaneously [4]. However,

the increase in temperature from 30 °C to 50 °C induces a decreased removal

of xylan content of 0.4% (from 1.5% to 1.9% in the residue). This temperature

944

8.3 Cold Caustic Extraction

0 20 40 60

0.0

1

2

3

30 .C 40 .C 50.C

Xylan content [%]

Time at Temperature [min]

Fig. 8.8 Influence of time and temperature during CCE treatment

of eucalyptus prehydrolysis-kraft pulp (E-PHK) at a constant

NaOH concentration of 70 g L–1 [28]. E-PHK: OZ pretreated,

kappa 0.6. CCE-treatment: 10%consistency , 30 min,

70 g NaOH L–1.

increase equally affects purification, as would a decrease in NaOH concentration

by 17 g L–1, from 70 to 53 g L–1, at 30 °C, respectively. The processability of the

CCE treatment is worsened at low temperature because washing is deteriorated

due to an increased lye viscosity. At a given washer capacity, this may result in

additional alkali losses. In industrial praxis, a compromise must be found between

economic considerations and pulp quality demand. In most cases, the temperature

level is adjusted to about 35 °C, which might fulfill both targets.

8.3.3

Presence of Hemicelluloses in the Lye

CCE treatment requires a comparatively high dosage of NaOH. Maintaining a

NaOH concentration of 80 g L–1 (74 kg NaOH t–1) at 10% pulp consistency

requires a total NaOH charge of 666 kg odt–1. The total alkali loss to the sewer is

economically by no means acceptable, however, and consequently methods to reuse

the entire quantity of the lye must be evaluated. In the case of cold caustic

purification of a PHK pulp, the excess lye may be completely recycled to the kraft

cook, provided that the demand of alkali for cooking is not lower than the amount

of alkali originating from the CCE treatment. In this particular case, white liquor

must be used as the alkali source. The efficacy of the white liquor with respect to

purification efficiency is equal to a pure NaOH if the strength of the white liquor

945