Volume.

Pulp Strength Delivery

Industrially produced softwood kraft pulps typically show only 60–80% of the

strength performance as compared to laboratory reference pulps. The strength

delivery is defined as the percentage of the tear index at a given tensile index from

an industrially cooked pulp in relation to the corresponding tear index of a laboratory

reference pulp from the same chip material. Extensive mill studies revealed

an average strength delivery from softwood batch cooking of 72% and 73%,

respectively [63,64]. The major reason for the low strength delivery of industrially

cooked pulps has been attributed to the hot blow which exerts a destructive effect

on the pulp fibers [22].The development of a more gentle pump discharge

improved the strength delivery to 84% as compared to only 70% using hot blow

discharge [65]. Surprisingly, the deleterious effect of hot blow on strength delivery

diminishes when cooking is performed according to the displacement technology,

as reported for the Superbatch technology. Mill trials at the Joutseno-Pulp Oy’s

digester house confirmed that when using liquor displacement cooking technique

the blow method made very little difference in the strength delivery. Blowing a

kappa-30 displacement batch cook without terminal displacement directly at cooking

temperature still resulted in about 90% strength delivery as compared to

100% in the case of a cold blow (see Tab. 4.29) [66].

256 4 Chemical Pulping Processes

Tab. 4.29 Strength delivery (percentage of tear index at 70Nm· g–1

tensile index to reference laboratory pulps) of industrially cooked

softwood pulps at different kappa number levels, and for both

cold and hot blow (according to [66]).

Properties Unit Cold blow Hot blow

Kappa level 30 22 30

Tear indexa mNm2 g–1 18 17 16

Strength delivery % 100 94 89

a) At tensile index 70 Nm· g–1

From the results shown in Tab. 4.29, it can be indirectly concluded that strength

delivery is improved from 72% (tear index 13 mN m2 g–1) to 89% (tear index

16 mN m2 g–1) by applying displacement technology. An additional reduction of

the severity of the discharge (e.g., by using cold pump discharge) further increases

the tear index from 16 to 18 mN m2 g–1. Thus, it can be concluded that by using

displacement cooking technology, pulps that are less vulnerable to the blow conditions

are produced. According to Tikka et al., the improved delignification selectivity

of the displacement batch cook is affected by the modified cooking chemistry,

in which black liquor with a high sulfidity seems to be the key factor [66]. The pretreatment

promotes selectivity and further reduces the required cooking H-factor

by 20–30%. The strength deficit is therefore due to a combination of cooking conditions

and the severity of discharge methods.

Residual Lignin Structures

The kappa number in unbleached and also bleached kraft pulps is made up of lignin

and other oxidizable structures denoted as false lignin. The latter contains

contributions from hexenuronic acid groups (HexA) and “non-lignin” structures

[67](one kappa number unit corresponds to 11.6 lmol HexA [68]). The amount of

true residual lignin can be measured directly using the Ox-Dem kappa number

method [69]. The contribution to the kappa number of the “non-lignin” structures

can then be calculated as the difference between the standard pulp kappa number

and the sum of the contributions of the residual lignin and HexA. It was shown

that about 24% of the kappa number in spruce kraft pulp and about 41% of the

kappa number in birch kraft pulp can be attributed to false lignin structures. In

absolute values, the false lignin accounts to approximately 5–6 kappa number

units for both softwood and hardwood kraft pulps [67]. In the softwood kraft pulp

the “non-lignin” structures dominate the false lignin fraction, whereas in the hardwood

kraft pulp HexA constitutes the main part of the false lignin (Tab. 4.30).

4.2 Kraft Pulping Processes 257

Tab. 4.30 Fractionation of pulp kappa number of spruce and

birch kraft pulps before and alter oxygen delignification

(according to [67]).

Pulp Kappa

number

Ox-Dem

Kappa

number

Kappa from

HexA

Kappa from

“non-lignin”

Spruce kraft pulp

Unbleached 22.5 17.2 1.3 4.0

O-delignified 10.7 4.6 1.2 4.9

Birch kraft pulp

Unbleached 13.8 8.1 4.7 1.0

O-delignified 9.6 3.2 4.3 2.1

Oxygen delignification is, in fact, a highly efficient delignification agent for

both softwood and hardwood pulps. Although the total kappa numbers indicate

only about 52% and 30% reduction, the degree of lignin removal is about 73%

and 60%, respectively. Table 4.30 also shows that the “non-lignin” structures are

formed during the kraft cook, and to some extent also during the oxygen delignification

stage. It has been speculated that these structures originate from carbohydrate

moieties which undergo elimination of water, with the final products being

aromatic structures containing catechols and chromones [70]. The elimination

reactions may result in keto groups in equilibrium, with the corresponding enol

structures being located along the carbohydrate chain, in for example, hemicelluloses.

Successive DMSO + 5% KOH extraction experiments revealed that a

decrease in the amount of xylan is accompanied by a near-equivalent decrease in

the amount of “non-lignin” structures, which suggests that the accessible part of

the xylan is the main source of false lignin [71]. With prolonged cooking, the content

of conjugated carbonyl groups in pulp dioxane lignin increases slightly [72].

The proportion of the glucose content in pulp dioxane lignin rises during cooking,

while the proportions of mannose and galactose decline and those of arabinose

and xylose remain rather constant. This might indicate that the pentoses form relatively

alkali-stable complexes with lignin [72].

The presence of alkali-stable chromophores based on carbohydrate structures

makes it likely that lignin–carbohydrate complexes (LCC) play an important role

in limiting the efficiency of oxygen delignification. The progressive removal of

hemicelluloses (e.g., xylan) from a Eucalyptus saligna prehydrolysis kraft pulp

(PHK) clearly results in a significant improvement of the oxygen delignification

efficiency. The kappa number of the unbleached pulps is adjusted to a comparable

level (9–12) by controlling the H-factor [73]. A selection of the most important

results is displayed in Tab. 4.31.

258 4 Chemical Pulping Processes

Tab. 4.31 Influence of prehydrolysis intensity, measured as

P-factor, on the efficiency of oxygen delignification

(according to [73]).

Cook # Cooking Process Unbleached Euca-PHK pulp OO-delignified Euca-PHK pulp

P-factor H-factor Yield

[%]

Kappa Viscosity

[mL g–1]

Xylan

[%]

Kappa D kappa

[%]