During HCE of spruce sulfite pulps appears to develop slightly more selectively

As compared to beech sulfite pulps (see Fig. 8.18). The reaction of purification

can be divided into two phases: first, a more-selective course; and second, a

less-selective course. Transition between the two phases appears for E and (E/O)

stages at R18 values of 95.5–96.0%, and in the case of (EO) treatment at R18 values

of 94.0–94.5%.

A yield loss of about 3% per 1% increase in alpha-cellulose content has been

reported elsewhere [4,27,30]. Recent studies on beech and spruce dissolving pulps

have confirmed this “rule-of-thumb” in general. However, small deviations are

Experienced as the yield loss is related to R18 content which, in contrast to alphacellulose

Or R10 values, is rather independent of viscosity in the range investigated.

A summary of the specific yield losses and NaOH consumption values is

Provided in Tab. 8.3.

The data in Tab. 8.3 show that HCE is very unselective at R18 values greater

than 96%. The one-stage hot purification and oxygen delignification behaves

slightly less selectively when exceeding R18 values of 94%. NaOH consumption is

A good indication for the degree of purification. Similar to kraft cooking,

Hot Caustic Extraction 955

92 94 96 98

HW-Sulfite SW-Sulfite:

E-stage EO-stage (E/O)-stage E-stage

Purification yield [%]

R18 content [%]

Fig. 8.18 Purification yield as a function of R18 content comparing

E-, (EO)-, and (E/O)-treatments of hardwood sulfite

Dissolving pulp (HW-S) and E-treatment of spruce sulfite

dissolving pulp (SW-S) [33]. Pulps and conditions are as in

Fig. 8.17.

Tab. 8.3 Specific yield losses and NaOH consumption values in

the course of E-, (EO), and (E/O) treatments of beech and

Spruce sulfite pulps.

Pulp Purification Yield loss per 1%

R18 increase

NaOHcons per

C6 sugar dissolved

<96% >96% <96% >96%

[% o.d. pulp] [mol mol–1]

HW-S E 3.7 5.0 1.4 1.6

HW-S EOa 4.0 2.0

HW-S (E/O) 3.2 5.0 2.0 2.0

SW-S E 3.3 4.1 1.4 1.6

a. Max. 95% R18.

the alkali consumption in pure E stages amounts to between 1.4 and 1.6 mol mol–1

Monosaccharide unit (calculated as C6) dissolved, indicating that the end products

Of degradation must be fragmented to smaller units than isosaccharinic acid, such

Pulp Purification

As glycolic, lactic, pyruvic and 3,4-dihydroxybutyric acids, as described by MacLeod

and Schroeder [34]. As anticipated, specific alkali consumption increases to a value

of about 2 mol mol–1 monosaccharide unit when oxygen delignification is integrated

Into the purification reaction, either in the same (EO) or in a separate stage

(E/O).

An elevated temperature between 80 and 120 °C is necessary to activate peeling

Reactions in the presence of sufficient alkali to achieve an increase in R18 and

R10, and a decrease in hemicelluloses. As shown in Fig. 8.19, cellulose degradation

begins at temperatures exceeding 140 °C, as suggested by a decrease in R10

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