Viscosity

[mL g–1]

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

30

40

50

60

70

80

90

Chain Scissions, 104/P

j

-104/P

0

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]).

1

30

40

50

60

70

80

90

5

Delignification efficiency [%]

Xylan content [%]

10

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 Wдnnstrцm [77]. Iversen and

Wдnnstrцm 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]).

Parameters Units MWLb) Hemlock kraft pulps

Conv. Kraft EMCC Kraft

Kappa number 26.8 26.0

Isolation yielda) % 14.2 49.5 46.7

Elemental composition

C % 61.0 65.2 62.8

H % 5.7 5.7 5.7

O % 32.9 27.9 29.8

S % 1.2 1.7

OCH3 % 15.3 12.1 10.5

Carboxylic groups mmol g–1 0.15 0.32 0.54

Hydroxyl Units

Aliphatic hydroxyls mmol g–1 4.27 2.14 2.15

Phenolic hydroxyls mmol g–1 1.15 2.71 2.50

Type A mmol g–1 0.02 0.02 0.01

Catechol (type B) mmol g–1 0.05 0.21 0.17

Guaiacol (type C) mmol g–1 0.62 0.23 0.56

Type D mmol g–1 0.06 0.38 0.27

Type E mmol g–1 0.35 1.64 1.32

a) Continuous dioxane acidolysis (dioxane-water = 85:15, 0.1 mol/l HCl).

b) Milled wood lignin from spruce.

R

OH

R

OH

OH

R

OH

OCH3

R

OH

OCH3

R

R

OH

R OCH3

(A) (B) (C) (D) (E)

262 4 Chemical Pulping Processes

The content of b-O-4-structures in residual lignin decreases with the extent of

delignification. The residual lignin in the EMCC pulp with kappa number 17.9

contained less b-O-4 structures and a higher content of C5 condensed structures

as compared to the residual lignin of conventional kraft pulp with kappa number

27.4 [85]. This is in accordance with the results obtained from the characterization

of residual lignins isolated from MCC and Super-Batch pulping technologies [86].

George et al., however, made different observations comparing residual lignins

isolated from spruce kraft pulps using also a dioxane acidolysis procedure [84].

The number of alkyl-O-aryl linkages, determined by 13C-NMR, was higher in the

modified residual lignin than in the conventional residual lignins. This observation

is in accordance with the higher amount of free phenolic groups present in

the conventional lignin. In a recent comparative study of conventional and laboratory-

simulated EMCC kraft pulps produced from Pinus elliottii, the residual lignin

of the latter had a higher content of b-O-4-structures and carboxylic groups. At

comparable kappa number, the amount of condensed structures was, however,

similar for both residual lignins [87].

The total phenolic hydroxyl content in the residual lignin continuously

increases during kraft pulping due to progressive cleavage of the b-O-4 bonds.

The guaicol-type of phenolic unit (type C) gradually decreases in parallel with the

progress in delignification. Conditions favoring the formation of unreactive carbon–

carbon bonds prevail, especially during conventional kraft cooking [88]. As

shown in Tab. 4.32, the amount of phenolic units substituted at the C5 position

(type E) continuously rise in both the dissolved and residual lignins. The Ca-C5

and the diphenylmethane units are described as the predominant C5 condensed

structures [89]. The formation of the diphenylmethane moieties has been

described as a considerably more facile reaction under soda pulping conditions as

compared to kraft pulping conditions. This may be one of the reasons why the

bleaching of soda pulps is more difficult compared to a kraft pulp at a given kappa

number [90]. Recently, the accumulation of completely unreactive 5–5′-biphenolic

hydroxyl groups was detected using quantitative 31P-NMR [91]. The final concentration

of the 5–5′structures after softwood kraft pulping was approximately

0.6 mmol g–1, and thus more than three-fold higher than the corresponding value

of 0.2 mmol g–1 detected for the milled wood lignin.

The molecular weight of the residual lignin increases slightly towards the end

of the cook, which may be an indication of progressive condensation reactions

[83]. Lignin from pulps and corresponding spent liquors during kraft pulping of

Pinus sylvestris covering the kappa number range between 116 and 17 were isolated

by acidic dioxane extraction and characterized by GPC, UV and IR spectroscopy

and oxidative degradation methods [72]. The average molar mass of both lignin

precipitated from the spent liquor and lignins isolated from pulps increases

with the progress in cooking. The lignins extracted from pulps showed a higher

molar mass as compared to the spent-liquor lignins.

In accordance with the higher content of phenolic hydroxyl groups, the conventional

kraft residual lignin exhibits a lower molecular mass than the modified residual

lignin at a given kappa number [84]. In extending the cook from kappa

4.2 Kraft Pulping Processes 263

number 30 to kappa number 15, the molecular weight of the modified residual

lignin continues to decrease, whereas that of the conventional residual lignin is

not influenced [84]. Since the number of phenolic hydroxyl groups in the case of

the residual lignins of both pulps remained constant, it may be assumed that rupture

of the ether bonds immediately leads to lignin dissolution. Extending the

conventional cook results in a significant decrease in the number of methoxyl

groups. This trend is less pronounced with modified cooks. The loss in methoxyl

groups may also be accounted for by a slight enrichment in p-hydroxyphenyl units

toward the end of the cook. It is known that the cleavage of alkyl-aryl ether linkages

is favored by the presence of methoxyl groups. Consequently, guaiacyl units

can be assumed to be removed prior to p-hydroxyphenyl units. In contrast to the

modified residual lignin, the content of quaternary carbons is significantly

reduced in case of the conventional residual lignin, which may be attributed to

the enrichment in p-hydroxyphenyl units.