Fig. 8.10 Effect of the presence of low (gamma) and high

(beta) molecular-weight hemicelluloses on xylan removal efficiency

During CCE treatment of hardwood acid sulfite dissolving

pulp (HW-S) at 100 g L–1 NaOH and 25 °C [28].

Unbleached HW-S: kappa number 5.7; total bleaching

sequence: CCE (E/O)ZP.

Pulp Purification

8.3.4

Placement of CCE in the Bleaching Sequence

The efficiency of cold alkali purification is reported to be improved by a preceding

hot caustic extraction stage in the case of a sulfite dissolving pulp [4]. More

Recently, it has been shown that the position of the CCE stage within a bleaching

Sequence has no significant impact on the degree of purification, provided that

Washing takes place between both purification stages (Fig. 8.11). In contrast,

When oxygen delignification (O) follows hot caustic extraction (E) without interstage

washing, denoted as (E/O) sequence, a CCE treatment preceding (E/O)

Seems to be advantageous over the reversed sequence with respect to delignification,

As illustrated in Fig. 8.11. The simple reason for the higher overall delignification

Efficiency of the latter is that unbleached pulp exhibits a higher level of

alkaline-extractable lignin than an (E/O) pretreated pulp, while the efficiency of

Oxygen delignification appears to be unaffected by the prehistory of pulp treatment.

Likewise, the position of CCE within a final bleaching sequence of a hardwood

TCF-bleached PHK pulp proved to have no influence on the purification efficiency,

As shown in Fig. 8.12. Nevertheless, placing CCE before the ozone stage

(Z) is preferred compared to both other alternatives because of a loss in viscosity

(in the case of CCE after Z) or higher capital costs (in the case of CCE after P).

Moreover, CCE treatment on the bleached pulp might be disadvantageous with

Untreated (E/O)-CCE CCE-(E/O)

Kappa number

Xylan content [%]

Xylan

CCE

(E/O)

(E/O)

CCE

Kappa number after First Stage after Second Stage

Fig. 8.11 Influence of the positions of CCE and

(E/O) stages in a sequential treatment on the

Purification and delignification performances

of a hardwood acid sulfite dissolving pulp [28].

Unbleached HW-S: kappa number 5.7; CCE

(E/O) versus (E/O)CCE; CCE-treatment:

100 g L–1 NaOH, 30 min, 30 °C; (E/O)-treatment:

E: 30 kg NaOH odt–1, 85 °C, 120 min; O:

85 °C, 90 min, pO2,t = 0 = 8 bar (abs).

Cold Caustic Extraction

Regard to possible impurities of the final product. It is also reported that pretreating

Pulp with cold alkali prior to hot caustic extraction reduces the amount of

alpha-cellulose being degraded during the latter process [11]. The reduction in viscosity

Loss is most pronounced when the pulp is partly converted from cellulose I

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