Appears to be alkali-resistant), the NaOH concentration must be increased from

0 100 200 300

0 100 200 300

C 50.C 80.C

Xylan content [%od]

Dissolved

Hemicellulose [% od]

NaOH concentration [g/l]

Fig. 8.1 Profiles of xylan content in the pulp

Residue (upper) and the amount of dissolved

Hemicelluloses (sum of beta- and gamma-cellulose)

(lower) during alkaline treatment of a

beech sulfite dissolving pulp (93.4%R18, 4.0%

xylan) at different temperatures [12]. Caustic

treatment: 5%consistency, 30 min reaction

time, NaOH concentrations: 20, 40, 60, 80,

100, 140, 180, 280, and 340 g L–1.

Pulp Purification

Reactions between Pulp Constituents and Aqueous Sodium Hydroxide Solution

100 g L–1 to about 140 g L–1 when raising the temperature from 20 to 50 °C. The

alkaline treatments at 50 °C and 80 °C reveal a comparable pattern of xylan

removal up to a lye concentration of about 280 g L–1. The xylan removal efficiency

remains unchanged at 80 °C and also at NaOH concentration up to 340 g L–1, but

Is slightly reduced at lower temperatures.

The profile of the amount of hemicelluloses dissolved during alkaline treatment

Resembles the swelling behavior of cellulose in dependence on lye concentration,

as experienced by Saito [13,14]. At low temperature (20 °C), the amount of dissolved

Hemicelluloses increases rapidly with increasing NaOH concentration, and

passes through a maximum at 100 g NaOH L–1. While the residual xylan content

Remains fairly constant with increasing lye concentration, the amount of dissolved

Hemicellulose decreases significantly to values less than half of the amount determined

At maximum solubility. In the low lye concentration range up to 170 g

NaOH L–1, the solubility of pulp constituents is significantly lower at 50 °C as compared

to 20 °C, whereas the maximum solubility is shifted to 140 g NaOH L–1. At

Higher NaOH concentrations, the pattern of the solubility of hemicelluloses develops

quite comparably for both temperatures, 20 °C and 50 °C, respectively. In contrast,

alkaline treatment at 80 °C causes a steady increase in hemicellulose solubility

up to a NaOH concentration of 280 g L–1. Beyond this lye concentration, the

Amount of dissolved hemicelluloses experiences a slight reduction (see Fig. 8.1,

lower). In hot alkali treatments (80 °C), the removal of short-chain carbohydrates

Is essentially governed by chemical degradation reactions involving endwise depolymerization

Reactions (the peeling reaction). With increasing temperature, the

Peeling reaction becomes the dominant pathway for the degradation of pulp carbohydrates.

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