Chain scissions

[0.1·mmol AHG–1]

Ka118 0 1057 47.7 11.7 934 14.5 6.6 43.6 772 1.041

Ka121 107 708 43.7 12.3 1411 8.7 6.4 48.0 1008 1.182

Ka124 1701 708 42.91 9.0 1152 6.9 4.41 51.1 865 1.269

Ka125 210 368 41.4 9.9 1353 5.2 4.1 58.6 957 1.295

Ka127 310 368 40.6 8.8 1304 3.8 3.4 61.4 936 1.288

Ka129 710 368 36.5 9.0 1112 2.2 2.6 71.1 848 1.244

Ka131 1900 368 31.6 11.1 668 1.3 2.0 82.0 578 1.191

Oxygen delignification was performed in a two-stage reaction without interstage

washing, with 15 min retention time in the first and 60 min in the second reactor,

respectively. The reaction temperature was kept constant at 110 °C throughout

both stages. The total alkali charge of 25 kg t–1, was added in the first stage. The

data in Tab. 4.31 indicate that the efficiency of oxygen delignification improves

along with the removal of the xylan content. Parallel with the reduction in the

hemicellulose content, the number of chain scissions increases until a residual

xylan content of approximately 5% is reached. When the residual xylan content is

further reduced to below 2%, the residual cellulose fraction again becomes more

resistant to degradation reactions (Fig. 4.57).

Interestingly, the degree of delignification during the oxygen delignification

stage is linearly correlated with the logarithm of the xylan content of the Eucalyptus

saligna prehydrolysis kraft pulp (Fig. 4.58).

Recently, the correlation between the residual amount of hemicelluloses and

delignification efficiency during oxygen delignification was confirmed for both

softwood and hardwood kraft pulps, with and without pre-hydrolysis [74]. Surprisingly,

the kappa numbers of the pulps after oxygen delignification display a very

similar final lignin content, expressed as Ox-Dem kappa. The kraft pulps without

pre-hydrolysis (paper-grade pulps) contain a considerably higher amount of “nonlignin”

and HexA structures as part of the kappa number as compared to the prehydrolysis

kraft pulps (dissolving pulps). As shown previously, the false lignin

fraction which is predominantly derived from carbohydrate structures is not susceptible

to oxygen delignification. On the contrary, during oxygen delignification

the proportion of “non-lignin” kappa number fractions even increases. The presence

of chemical linkages between cellulose, the residual hemicellulose and

the residual lignin in native wood were reported by Isogai et al. [75], and the

4.2 Kraft Pulping Processes 259

0 2 4 6 8 10 12 14 16

Chain Scissions, 104/P

j

-104/P

Degree of delignification

Degree of Delignification [%]

Xylan content [%]

1.0

1.1

1.2

1.3

1.4

Chain scissions

Fig. 4.57 Influence of the residual xylan content of a Eucalyptus

saligna prehydrolysis kraft pulp on the delignification efficiency

and number of chain scissions in a subsequent oxygen

delignification stage (OO: 15/60 min, 110 °C, 25 kg NaOH t–1)

(according to [73]).

Delignification efficiency [%]

Xylan content [%]

Fig. 4.58 Influence of the residual xylan content of a

Eucalyptus saligna prehydrolysis kraft pulp on the delignification

efficiency in a subsequent oxygen delignification stage

(OO: 15/60 min, 110 °C, 25 kg NaOH t–1) (according to [73]).

260 4 Chemical Pulping Processes

formation of alkali-stable ethers and carbon–carbon linkages during kraft pulping

were reported by Ohara et al. [76]and Gierer and Wannstrom [77]. Iversen and

Wannstrom proposed the alkali-catalyzed formation of ether bonds between carbohydrate

hydroxyl groups and lignin oxiranes derived from the degradation of the

lignin molecule during kraft pulping [78].

The most prominent lignin structures, which are responsible for the reactivity

in subsequent bleaching treatments, are the alkyl-aryl ether linkages (b-O-4-structures),

the methoxyl groups, the aliphatic and aromatic hydroxyl groups and the

hydrophilic substituents, such as carbonyl and carboxylic groups [79]. Moreover,

the macromolecular properties of the residual lignin provide additional information

about the conditions during the delignification reactions. Unfortunately,

there is still no method for the isolation of a representative residual lignin of

unchanged physical and chemical structure. The acidolytic and enzymatic hydrolysis

methods are used for the isolation of residual lignin. Additionally, a combination

of enzymatic and acidic hydrolysis as a two-step procedure was proposed [80].

The latter shows some advantages with respect to the yield and the amount of

impurities in comparison to the one-step procedure. The dioxane acidolysis,

which is still the most common method, produces pure lignin of only about 40%

yield. Unfortunately, the b-aryl-ether and lignin–carbohydrate linkages are cleaved

during the isolation procedure, which is seen as a reduction in the molecular

weight of the lignin and in an increased phenolic hydroxyl group content [81].

According to Gellerstedt et al., the formation of condensed phenolic groups during

acidolysis is not probable [82]. Although residual lignin can be recovered

quantitatively after enzymatic hydrolysis, the isolated lignin contains large

amounts of impurities which aggravate structural lignin characterization to a significant

degree.

There are some indications that modern modified cooking technologies alter

the structure of residual kraft lignin beneficially for subsequent bleaching treatments.

The residual lignin isolated from a hemlock EMCC kraft pulp using a

dioxane acidolysis protocol shows a lower amount of condensed phenolic and

higher amounts of carboxylic acids and uncondensed phenolic units as compared

to the residual lignin structure from a conventional hemlock kraft pulp [83]. Comparative

data from lignin characterizations are listed in Tab. 4.32.

The enrichment of carboxylic groups during kraft cooking is followed by the

elimination of aliphatic hydroxyl groups, which are decreased from 4.27 mmol g–1

in case of the milled wood lignin to 2.14 resp. 2.15 mmol g–1 for the residual lignin

isolated from the hemlock unbleached kraft pulps (Tab. 4.32). This is in agreement

with the growing elimination of the a-hydroxyl groups present in b-O-4

ether units. The relatively high content of primary hydroxyl groups in the wood

lignin can be expected to be diminished during pulping because of the known

reactions in which the c-carbon is eliminated as formaldehyde. The content of the

primary hydroxyl groups is significantly higher in the residual lignin isolated

from the conventional spruce kraft pulp as compared to the residual lignin from

modified kraft pulp (0.24 mol per aromatic unit versus 0.33 mol per aromatic

unit, respectively) [84].

4.2 Kraft Pulping Processes 261

Tab. 4.32 Comparative evaluation of the residual lignin

structures isolated from milled wood lignin, unbleached

conventional and EMCC kraft pulps (according to [83]).

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