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

[%]

Дата добавления: 2015-10-21