This explains the different pattern of hemicelluloses removal as compared

to the alkaline treatment at lower temperatures (20 °C and 50 °C). In contrast,

cold alkali treatment at 20 °C induces intermicellar and intramicellar swelling,

Permitting short-chain material to dissolve. The physical interaction between

Cellulose and aqueous sodium hydroxide proceeds in several steps. According to

Bartunek [15] and Dobbins [16], the addition of low amounts of electrolytes (e.g.,

NaOH) seems to create unbound or “monomeric” water by shifting the equilibrium

Between clustered and free water. Swelling can thus be explained by the penetration

Of the unbound water molecules into the cellulose structure, while destroying

Intermolecular hydrogen bonds. Moreover, swelling facilitates the accessibility

Of the hydrated ions into the crystalline structure. The degree of swelling is governed

By both the number of water molecules present as hydrates of the alkali

Ions entering the cellulose structure, which decreases with increasing lye concentration,

And the penetration depth of these alkali ions into the structure, which

Increases with lye concentration until the conversion to alkali cellulose is completed.

Thus, swelling passes through a maximum at a lye concentration that is

Sufficient to ensure complete penetration of the whole structure. The decrease in

Swelling beyond this value can be explained by a disproportionally large reduction

Of the hydration number when further increasing the NaOH concentration.

It can be assumed that the extent of hemicellulose dissolution proceeds parallel

To the swelling behavior of the pulp. The monomeric sugar composition of the

Pulp Purification

0 100 200 300

γ

[% of total hemi removed]

Proportion of Xylan removed

Hemicellulose removed [%od]

NaOH concentration [g/l]

Gamma-Cellulose Fraction: Glucose Xylose Mannose degraded

Beta-Cellulose Fraction: Glucose Xylose Mannose

C

Dissolved as Xylan total removed Xylan

0 100 200 300

γ

Hemicellulose removed [%od]

NaOH concentration [g/l]

Gamma-Cellulose Fraction: Glucose Xylose Mannose Degraded

Beta-Cellulose Fraction: Glucose Xylose Mannose

C

[% of total hemi removed]

Proportion of Xylan removed

Dissolved as Xylan total removed Xylan

0 100 200 300

C

γ

[% of total hemi removed]

Proportion of Xylan removed

Hemicellulose removed [%od]

NaOH concentration [g/l]

Gamma-Cellulose Fraction: Glucose Xylose Mannose Degraded

Beta-Cellulose Fraction: Glucose Xylose Mannose

Dissolved as Xylan total removed Xylan

Fig. 8.2 Profiles of carbohydrate composition

Of the gamma- and beta-celluloses fractions dissolved

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