Change in viscosity is more pronounced for beech dissolving pulp, indicating that

The molecular weight of the removed hemicelluloses is lower than that from

Spruce dissolving pulp. Alternatively, a greater amount of low molecular-weight

Material is removed from the beech dissolving pulp during HCE treatment; this

Suggestion would be in line with the higher specific yield loss when compared to

Spruce dissolving pulp.

91 93 95 97

-1,6

-1,2

-0,8

-0,4

0,0

HW-Sulfite SW-Sulfite

Medium viscosity Medium viscosity High viscosity

Chain scissions, 1/DP

HCE

-1/DP

Untreated

[*10-4 mol/AGU]

R18 content [%]

Fig. 8.22 Change in pulp viscosity, expressed

As negative number of chain scissions, as a

Function of the degree of purification, characterized

As R18 content, during E-treatment of

Hardwood and softwood sulfite dissolving

pulps (HW-S, SW-S) [33]. Medium-viscosity

HW-S: viscosity 590 mL g–1, kappa number 4.6;

Medium-viscosity SW-S: viscosity 625 mL g–1,

kappa number 4.6; High-viscosity SW-S: viscosity

890 mL g–1, kappa number 12.2.

Hot Caustic Extraction 959

Purification versus Kappa Number and Extractives

Hot caustic purification also removes other pulp impurities such as lignin and

Extractives. Most of the kappa number reduction occurs already at low NaOH

Charge, and this can be attributed to a readily available lignin (Fig. 8.23). The

More alkali-resistant lignin is gradually decreased with increasing NaOH charge.

It may be speculated that part of the removed lignin is associated with the

Extracted (and degraded) xylan.

0 30 60 90 120

E-treatment of HW-Sulfite Pulp

Kappa number

NaOH charge [kg/odt]

Fig. 8.23 Course of kappa number as a function of NaOH

Charge during E-treatment of hardwood sulfite dissolving

pulps (HW-S) [33]. HW-S: viscosity 580 mL g–1, kappa number

5.1; E conditions: 90 °C, 240 min.

Hot caustic extraction is a very efficient stage in the removal of resin constituents

of sulfite pulps [27]. The saponification of fats, waxes and other esters is the

Key reaction responsible for the removal of extractives. The removal efficiency can

Be further enhanced by the addition of surfactants (nonyl phenol with attached

Polyoxyethylene chain), and this may also solubilize the nonsaponifiables. The deresination

Of softwood pulps with large amounts of resin can be further improved

By subjecting the pulp to increased mechanical forces that allow removal of the

Encapsulated resin from the ray cells. A process developed by Domsjo involves the

Use of a Frotapulper, along with the addition of caustic for the de-resination of sulfite

pulps (Fig. 8.24) [35,36].

Pulp Purification

0 10 20 30 40 50

0.0

0.2

0.4

0.6

0.8

DCM extractives, %

Energy, kWh/odt

Fig. 8.24 Course of dichloromethane (DCM) extractives in

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