Viscosity

[mL g–1]

DKappa/CS*

[kg odt–1] OXE R OXE

OZQP1P2 Unbleached

O

Z

QP1

P2

O3

H2O2

H2O2

5.2

25

25

650

1470

1470

650

2120

3590

27.3

15.5

9.9

3.6

2.8

46.7

77.5

83.9

955

895

800

558

546

9.1

3.6

3.6

OQP1ZP2 Unbleached

O

QP1

Z

P2

H2O2

O3

H2O2

25

5.2

25

1470

650

1470

1470

2120

3590

27.3

15.5

7.1

2.7

1.0

70.3

77.6

88.9

955

895

807

730

665

14.9

10.8

7.8

O: 10% consistency, 0.58 Mpa, 90 °C, 60 min.

Z: 10% consistency, 2 °C, pH 2.2.

P: 10 % consistency, 3% NaOH, 0.05% MgSO4, 0.2% DTPA,

85 °C, 240 min.

CS = chain scissions given as 104

Pt _ 104

PO _ in 10–4 mol AGU–1.

These results indicate that the residual chromophore structures are activated by

ozonation towards a subsequent alkaline hydrogen peroxide bleaching, presumably

by introducing additional phenolic hydroxyl groups [130]. The presence of an

OH in the ortho- or para-position to the a-C of the side chain containing a keto

group makes this group susceptible to alkaline hydrogen peroxide, where the aro-

848 7Pulp Bleaching

matic ketone structure is converted to a phenol according to a Dakin reaction. In

subsequent oxidation reactions, the phenols are further oxidized to aliphatic carbonic

acids.

Table 7.44 also shows that the overall selectivity of an OQP1ZP2 sequence is better

as compared to an OZQP1P2 treatment, presumably because of the introduction

of more alkaline-labile groups during the Z treatment directly after an oxygen

stage, than after an alkaline hydrogen peroxide step with a significantly lower

kappa number prior to ozonation.

7.6

Hydrogen Peroxide Bleaching

Hans-Ullrich Sьss

7.6.1

Introduction

In 1818, J. L. Thenard discovered hydrogen peroxide (H2O2) by reacting barium

peroxide with nitric acid [1]. Based on this reaction, the commercial production of

H2O2 began around 1880 [2]. The very diluted (~3%) H2O2 produced by the barium

process found only limited use due to the high production costs and a poor stability.

However, the advantages of H2O2 in bleaching were rapidly recognized, it was

applied for example in the bleaching of precious products such as ivory. The disadvantages

of the barium process were overcome by the electrochemical process,

which was based on the electrolysis of a diluted sulfuric acid solution and subsequent

hydrolysis of the peroxy disulfuric acid to H2O2 and sulfuric acid [3]. The

electrochemical process allowed the production of pure and stable, more highly

concentrated (~30%) H2O2 solutions. The first commercial H2O2 plant using the

electrochemical process started production in 1908 at Österreichische Chemische

Werke, Weissenstein, Austria.

For the major part of the twentieth century, sodium peroxide played a more

important role than H2O2 in bleaching applications. Its relatively simple production

from sodium metal by air oxidation was the cheaper route to a peroxygen

compound. On dilution in water, it yields a strong alkaline solution of hydrogen

peroxide, which could be applied directly in bleaching processes:

Na2O2 _ 2H2O _ 2NaOH _ H2O2 _106_

For the majority of processes, the high alkalinity is a disadvantage, and therefore

acid had to be added to achieve a partial neutralization. This, and the more

complicated dissolution of the solid compound Na2O2 in contrast to the simple

addition of the liquids caustic soda and hydrogen peroxide to a bleaching process,

resulted in a slow phasing out of sodium peroxide as a bleaching chemical. The

development of the anthraquinone process (the so-called AO process) in the mid-

1930s at BASF [4,5] resulted in a more economical pathway to hydrogen peroxide

7.6 Hydrogen Peroxide Bleaching 849

compared with the electrochemical reaction. In 2003, the worldwide capacity for

H2O2 production was estimated as 3.3 million tonnes, based on different variations

of the AO process. The predominant proportion of H2O2 is used in bleaching

processes.

7.6.2

H2O2 Manufacture

The anthraquinone process for H2O2 production starts with the catalytic hydrogenation

of a 2-alkyl-9,10-anthraquinone. The resulting hydroquinone is oxidized

with oxygen, usually air, to yield H2O2 and the corresponding quinone. After separation

of H2O2 by extraction with water, the quinone is recycled within the process

to the hydrogenation step [6]. The hydrogenation is dominantly made with palladium

as catalyst, applied either as palladium black or supported on a carrier for

slurry or fixed-bed operation. Several alternatives for the alkyl side chain are in

commercial use. The patent literature cites 2-ethylanthraquinone, 2-tert-butylanthraquinone,

mixed 2-amylanthraquinones, and 2-neopentylanthraquinone.

These compounds differ in solubility in the so-called “working solution”. Because

quinone and hydroquinone have different solubility, solvent mixtures are mostly

used. Quinones dissolve well in nonpolar aromatic solvents, whereas hydroquinones

dissolve better in polar solvents. In order to avoid losses of the active compounds,

hydrogenation selectivity is important and a regeneration of the working

solution is required.

Commercial H2O2 solutions are prepared by purification and concentration

steps. Hydrogen peroxide is available as a clear, colorless solution which has a specific

odor and is completely miscible with water. The solutions are stabilized by

acidification with phosphoric acid and the addition of stannate and small amounts

of chelants. A typical stabilizer is 1-hydroxy ethylene 1,1-diphosphonic acid

(HEDP). Hydrogen peroxide is stored in stainless steel or aluminum or polyethylene

tanks. For storage and handling, local legislation must be considered.

For industrial applications in bleaching processes, H2O2 is stored typically in solutions

with a concentration between 50% and 70%. It may be diluted before its

addition to the pulp. If effective mixing is guaranteed, an undiluted addition is

possible.

7.6.3

Physical Properties

Hydrogen peroxide is generally supplied as an aqueous solution, typically in concentrations

between 35% and 70% by weight. These acidic solutions of H2O2 in

water are very stable. Hydrogen peroxide can be stored for months in stainless

steel tanks, without significant changes of the content. Some physical constants

of H2O2 are listed in Tab. 7.45. The main commercial grades are those containing

between 50% and 70% H2O2 by weight.

850 7Pulp Bleaching

Tab. 7.45 The physical properties of commercial hydrogen

peroxide (H2O2) solutions.

Concentration (by weight) Boiling pointa

[°C]

Melting point

[°C]

Densityb

[g cm–3]

100% H2O2 150.2 –0.42 1.443

70% H2O2 125 –40 1.288

60% H2O2 119 –56 1.241

50% H2O2 114 –52 1.196

Water 100 0 0.997

a. Extrapolated values because decomposition will reduce boiling

point continuously.

b. 25 °C.

Fig. 7.116 Configuration of hydrogen peroxide in the solid phase.

The bond length between the two oxygen atoms of the H2O2 molecule is rather

long (Fig. 7.116). Compared to water, the energy content of H2O2 is much higher.

For water, the heat of formation (DH) [Eq. (107)] from the elements is as low as –

286 kJ mol–1, whereas for H2O2 [Eq. (108)] the corresponding value is only –

188 kJ mol–1 [7]. In consequence, H2O2 is less stable and can disproportionate into

water and oxygen:

H2 _ 0_5O2 _ H2O DH _ _286 kJ mol_1 _107_

H2 _ O2 _ H2O2 DH _ _188 kJ mol_1 _108_

Since the activation energy for the cleavage of the oxygen–oxygen bond is rather

low (DH = –71kJ mol–1) [7], traces of contaminants can start this reaction. Basically,

the decomposition is a redox process, with H2O2 either supplying electrons

and yielding oxygen, or accepting electrons and yielding water. Metal salts of different

states of oxidation can start the decomposition reaction. The first step can

be the reduction according to Eq. (109):

7.6 Hydrogen Peroxide Bleaching 851

2Me2_ _ H2O2 _ 2Me_ _ O2 _ 2H_ _109_

The alternative is the oxidation of a metal according to Eq. (110):

2Me_ _ H2O2 _ 2H_ _ 2Me2_ _ 2H2O _110_

The reaction certainly can also start with the reduced form of metal. The overall

reaction is identical, it being the formation of water and oxygen from H2O2 with

the redox system of the metal is acting as the catalyst [8].

The decomposition of H2O2 is, in addition, catalyzed by alkali, with the reaction

steps being as follows:

H2O2 _ OH_ _ H2O _ HOO_ _111_

HOO_ _ H2O2 _ H2O _ O2 _ OH_ _112_

Since bleaching with H2O2 requires alkaline conditions, this decomposition

reaction is very important for its technical application.

Single electron transfer reactions of H2O2 with catalysts yield radicals, these

decomposition reactions taking place with either metals or with enzymes (e.g.,

catalase). Radical formation may also be the result of a thermal cleavage of the

oxygen–oxygen bond:

H2O2 _ Me_ _ OH_ _ _OH _ Me2_ _113_

H2O2 _ _OH _ H2O _ _OOH _114_

H2O _ _OOH _ _OO_ _ H3O_ _115_

The hydroxyl radical, the hydroperoxy radical, and the superoxide anion radical

are important intermediates. Each of these cause side reactions in bleaching processes,

with delignification as a positive and depolymerization of the cellulose as a

negative result. In general, radicals produce more negative effects than positive

results on delignification. Therefore, if present in higher amounts, transition metal

ions must be removed by acid washing or “neutralized” by chelation before and

during a peroxide treatment.

Tab. 7.46 Standard oxidation potential for hydrogen peroxide [7].

Reaction pH Oxidation potential

[E°/V]

H2O2 + 2H+ +2e– 2H2O 0 1.776

HO2

– + H2O + 2e– 3 OH– 14 0.878

852 7Pulp Bleaching

The oxidation potential for H2O2 is significantly higher under acidic conditions

(Tab. 7.46). Despite this, typical bleaching reactions are conducted under alkaline

conditions. Formation of the perhydroxyl anion [Eq. (111)], a nucleophile intermediate,

is responsible for the oxidation of chromophores in lignin through the

cleavage of side chains [9]. The effect of a H2O2 treatment is dominantly an

increase of the brightness. Delignification with H2O2 is to a large extent the result

of the action of the radicals produced in Eqs. (112–115) [10]. At moderate temperature,

under buffered conditions, and in the absence of transition metals, the

delignifying effect of H2O2 is limited. The perhydroxyl anion, being a nucleophile,

cannot attack the electron-rich aromatic rings of the residual lignin. Consequently

a degradation of polymerized lignin, which can be the result of high-intensity

pulping conditions will not occur in H2O2 bleaching.

Under acidic conditions, H2O2 reacts only slowly with organic compounds. At

high temperature, the hydroxylation reactions that may occur do not result in any

bleaching effect; on the contrary, the reaction might generate new chromophores

(phenols to quinones, etc.). Because peracids have a better leaving groups, their

reaction is both more rapid and more selective. For oxidation under acidic conditions

therefore, peracids such as peracetic acid are the preferred reaction partners.

7.6.4

Chemistry of hydrogen peroxide bleaching

Manfred Schwanninger

Although the major fraction of wood lignin can be removed by pulping, the

remainder of the lignin (residual lignin) is rather resistant under the pulping conditions.

In order to remove the residual lignin from the pulp, oxidative lignin degradation

with bleaching reagents such as dioxygen, H2O2, ozone, and chlorine

dioxide is required. Hydrogen peroxide is mainly used to brighten pulps (removal

of chromophores) during the final bleaching stages, and at the end of a conventional

bleaching sequence to prevent the pulp from losing brightness over time.

Carbonyl carbons or the vinylogous carbon atoms in intermediates of the enone

type (quinone methide intermediate; see Section 4.2.4, Chemistry of kraft pulping,

Scheme 3) are the locations where the nucleophile (the hydroperoxy anion)

begins the attack [11,12]. The hydroperoxy anion is incapable of degrading polymerized

lignin directly via an attack of the electron rich aromatic rings of the residual

lignin, but by cleaving the sidechain i.e. Dakin and Dakin-like reactions the

lignin can be depolymerised.

The parameters that influence bleachability, the composition of lignin and residual

lignin after cooking and their reactivity, as well as the composition of residual

lignin–carbohydrate complexes (RLCC) before and after oxygen bleaching, the

influence of inorganic substances and their role in the protection/degradation of

cellulose, have been described previously.

Hydrogen peroxide and the hydroperoxy anion respectively evolve in situ [13]

during oxygen bleaching. In contrast to dioxygen, which contains multiple bonds

between the O atoms, H2O2 has only one bond, and this can be easily broken.

7.6 Hydrogen Peroxide Bleaching 853

Under the conditions used in H2O2 bleaching, with the pH in the range of 10–12,

the standard redox potentials of the reactive species are substantially reduced

(Scheme 7.36) due to the lower potential of the ionized form. Hydrogen peroxide

(hydroperoxy anion) can either be oxidized by a one-electron step to the hydroperoxyl

radical (superoxide anion radical), or reduced to the hydroxyl radical (oxyl

anion radical) (Scheme 7.36).

O2

+e-, H+

HOOH H2O+ 2 H2O

pKa= 4.8 11.6 11.9

O2

-

H++ H++HOO- O- H+ +

+e-, H+

+e-, H+ +e-, H+

E0 at pH 14 - 0.33 0.20 - 0.03 1.77

Dioxygen

Hydroperoxyl

radical

Hydrogen

peroxide

Hydroxyl

radical

Superoxide

anion radical

Hydroperoxy

anion

Oxyl anion

radical

Oxygen species

Anionic form

45.71

Hydroxide

ion

OH-

HOO HO Water

Scheme 7.36 Dioxygen reductions proceeding in four consecutive

one-electron steps (E0 standard reduction potential)

(1According to [14]).

The actual concentration of the hydroperoxy anion depends on the pH of the

solution (Scheme 7.36) and, of course, on the amount of H2O2 added. The pH value

is not the best measure to determine the effective hydroperoxy anion concentration,

however, because of the interaction of the OH– ion and H2O2, different

solutions where either component is in excess might have the same pH and yet

have a 10-fold difference in hydroperoxy anion concentration [15]. Conversely, two

solutions may give the same approximate concentration of hydroperoxy anions

and have different pH values [15]. Notably, in this very interesting study [15] it was

also found that, during H2O2 bleaching of cotton cellulose, the latter acted as a stabilizer

for the peroxide.

7.6.4.1 Decomposition of H2O2

Transition metal ions such as copper, manganese, and iron can react with H2O2 in

a Fenton-type reaction:

HOOH + Me(n – 1)+ →Men+ + HO_ + HO–.

In this reaction, a homolytic cleavage of the O–O bond occurs, generating hydroxide

ion (OH–) and the hydroxyl radical (OH_), with the latter possibly being

formed via an oxoiron(IV) intermediate [16]. The peroxide can also be effective as

an oxidant, and in a transition metal-induced cleavage of the H–OO bond the

hydroperoxyl radical (HOO_) is formed:

HOOH + Me3+ →Me2+HOO_ + H+.

854 7Pulp Bleaching

A thermal homolytic cleavage of H2O2 also occurs:

HOO– + HOOH→(energy) HO2_ + HO_ + HO–.

The stabilizing effect of magnesium on H2O2 has long been known [17], and has

been confirmed in several studies [18,19]. Different possible explanations for the

protective effects of magnesium compounds reported by Reitberger et al. [20]

were substantiated by others (see Section 7.3.2.7, Chemistry of oxygen delignification).

The lifetime (half-life) of H2O2 in different aqueous systems under various

chemical additions (NaOH, magnesium sulfate, DTPA) in the presence and

absence of fully bleached softwood kraft pulp (FBSKP) was increased significantly

by the addition of magnesium sulfate and DPTA [21]. The details listed in

Tab. 7.47 show a persistently longer half-life for the acid-treated pulp (FBSKP-A).

In the presence of FBSKP, MgSO4 addition lengthened the peroxide half-life significantly,

from 8 to 36 min, while Mg in a chelated form (Mg + Q) performed

even better, increasing the half-life to 83 min. Compared to the results of Mg and

DTPA alone, a synergistic effect for complexed Mg can be claimed [21]. The differences

between FBSKP-A (half-life 22 min) and FBSKP (3 min) are attributable to

higher concentrations of transition metals, particularly manganese, in the FBSKP

(4.3 ppm compared to 0.3 ppm in the FBSKP-A) [21].

Kadla et al. [aa] subjected a technical pine kraft lignin to alkaline hydrogen peroxide

oxidation at various temperatures. In the absence of DTMPA (diethylenetriaminepentamethylene-

pentaphosphonic acid) the hydrogen peroxide was rapidly

degraded, and accompanied by only minimal lignin oxidation. In the presence of

Tab. 7.47 Half-lives of hydrogen peroxide (t1/2, min), obtained for

the different aqueous systems and various chemicals additions

(OH = 2% NaOH; P = 2% H2O2; Mg = 0.05% magnesium

sulfate; Q = 0.2% DTPA-Na; T = 363 K) (from Ref. [21]).

System With pulp Without pulp

FBSKP-Aa FBSKP Water

I (OH + P) 22 ｱ18.0 ｱ0.4 41 ｱ24

II (OH + P + Q) 25 ｱ3 12.2 ｱ0.4 130 ｱ40

III (OH + P + Mg) 190 ｱ50 36.0 ｱ13.0 300 ｱ130

IV Grp 1(Mg + Q)

+ Grp 2 (OH + P)

240 ｱ150 83.0 ｱ11.0

1330 ｱ590

V Grp 1(Mg + OH)

+ Grp 2 (P + Q)

370 ｱ130 39.0 ｱ13.0

a. Acid-treated at pH = 1.5.

7.6 Hydrogen Peroxide Bleaching 855

DTMPA (stabilize H2O2 at high temperatures and alkali [bb]) the lignin undergo

increasing levels of oxidation and degradation with increasing temperature. The

highest degree of selectivity was observed at 90 °C, i.e. the highest amount of phenolic

hydroxyl groups degraded and the highest amount of lignin degraded as a

function of hydrogen peroxide consumed. The highest amount of lignin degradation,

over 80%, occurred at 110 °C. Analyses of the degraded lignins indicated that

both phenolic and nonphenolic lignin moieties were degraded [aa].

7.6.4.2 Residual Lignin

The sites of nucleophilic attacks in lignins are shown in Fig. 7.116. By elimination

of an a– (see Section 4.2.4, Chemistry of kraft pulping) or, in conjugated structures,

a c-substituent, a quinone-methide intermediate is formed from the arylalkane

unit (Fig. 7.116), which involves the loss of two electrons, and results in the

generation of centers of low electron density (d+) that constitute the sites of attack

by nucleophiles [22].

O

R2 OCH3

CH





arylalkane unit

R1 = OH, OAr or OAlk

arylpropene unit

quinone-methide intermediate

O

R2 OCH3

CH

HC

CH2

O

R2 OCH3

C

C

CH2

R

O

R3

-carbonyl group containing

R = OAr, Ar or Alk















C C C C O



Fig. 7.116 Sites of nucleophilic (d+) attacks in lignin (adapted from Ref. [22]).

A nucleophilic attack starts with the addition of the hydroperoxy anion to carbonyl

and conjugated carbonyl structures (Scheme 7.37, 1) giving a hydroperoxide

(2) which forms an epoxide (4). After an additional nucleophilic attack the Ca–Cb

bond will be cleaved.

C

C

C

O

HOO-

HOO

C

C

C

O-

O

C

C

C

O-

HO

C C C

O O

- OH -

1 2 3 4

Scheme 7.37 Formation of hydroperoxide via a nucleophilic reaction.

The hydroperoxide anion adds rapidly to quinoid structures (Scheme 7.38). By

addition to an ortho-quinone (5) hydroperoxides (6, 9) are formed, leading to the

formation of dioxetane (7) or oxirane (10) intermediates followed by cleavage of

856 7Pulp Bleaching

O

R1 O

O

OO- R1

+ HOO -

- H+ O-

-O

R1 O

O-

O O

OR1 -

O

O-

O

R1 O

+ HOO-

O

O- R1

OOH

- HO - + HOO-

O

R1 O

O

O

OR1 -

O

O-

O

+ HOO -

+ OH -

degradation

products

O

R1 OCH3

O

+ HOO -

- H+

O

OCH3

R1

O-

OO-

-O

OCH3

R1

O-

O

O O

O

R1

O-

O-

O-

+ H2O

- CH3OH

+ HOO-

+ OH -

degradation

products

O

R1 OCH3

CH

+ HOO-

O-

R1 OCH3

HC O OH

- OH -

O

R1 OCH3

HC O

+ HOO-

- OH -

O

R1 OCH3

O

+ HOO -

+ OH -

degradation

products

O

R1 OCH3

CH

CH

[ O- ]

C

H O

+ HOO -

O

R1 OCH3

CH

CH

[ O- ]

C

O- H

HOO

- OH -

O

R1 OCH3

CH

CH

[ O- ]

C

H O

O

+ HOO-

O

R1 OCH3

C

[ O- ]

O H

HCOO-

+ HOO-

+ OH -

degradation

products

O

R1 OCH3

C

C

CH2

+

+ HOO-

O

OR

O

R1 OCH3

C

C

CH2

O-

OR

HOO

O

R1 OCH3

C

C

CH2

O

OR

O

- OH - + HOO -

O

R1 OCH3

C

O O-

HCOO-

+

+

ROH

H3CO

R1

R =

5 7 8

5 10 11

12 13 14 15

16 17 18 19

20 21 22 23

24 25 26 27

HC O

+

CO2

CO2

6

9

Scheme 7.38 Addition of hydroperoxide anions to quinoid

structures and to side-chain enone structures (adapted from

Refs. [12,23]).

7.6 Hydrogen Peroxide Bleaching 857

the ring giving dicarboxylic acids (8, 11) that can be further degraded. Adding the

hydroperoxide anion to a para-quinone with a methoxyl group (12) gives via a

hydroperoxide (13), a dioxetane (14), and the ring is cleaved after demethoxylation,

giving a dicarboxylic acid (15). The hydroperoxide (17) formed after hydroperoxide

anion addition to an arylalkane (quinone methide structure) (16) leads to an oxirane

(18). A further nucleophilic attack cleaves the bond between the Ca-atom and

the ring, thereby forming an aldehyde group and a para-quinone (19) which can

be further degraded (12–15).

Side chains with enone structures (20, 24) also afford hydroperoxides (21, 25)

and subsequently oxirane intermediates (22, 26), leading to cleavage of the Ca–Cb

bonds and producing an aldehyde (23) or carboxylic acid (27) at the aromatic ring

and carboxylic acids groups on the split-off residues.

Phenylpropanols and phenylpropanones (Scheme 7.39, 28) react with the

hydroperoxide anion to form a hydroperoxide (29) that is rearranged to an ester

(30) which can be cleaved to an aldehyde and a phenolate (31) in a Dakin-like reaction.

The latter can be oxidized to a para-quinone (32) and further degraded (see

Scheme 7.38, 12–15).

In a lignin model study, guaiacylglycerol-b-guaiacyl-ether was oxidized with

alkaline H2O2 in the presence of pulp in order to simulate technical bleaching conditions

[24]. The phenolic b-O-4 structure was found to react rather rapidly with

H2O2 and, from the mixture of products formed, it was concluded that the main

reaction was a side-chain displacement that proceeded via the so-called Dakin-like

mechanism. This was followed by secondary reactions that resulted in cleavage of

the molecule, accompanied by an extensive formation of carboxyl groups [24].

O-

OCH3

C

R

-OOH

O

28

O-

OCH3

C

R

O-

29

O

OH

O-

OCH3

C

R

O

30

O-

OCH3

31

+

O-

Ox

O

OCH3

32

O

further oxidation

giving aliphatic

degradation products

O

Ox - OH -

C

R

- O O

+ 2OH-, - H2O

Scheme 7.39 Dakin reaction at the Ca-keto group of a phenolic

unit (adapted from Ref. [23]).

A bleaching sequence involving oxygen bleaching (O), treatment with a chelating

agent EDTA (Q), and an alkaline H2O2 stage (P), showed that partial removal

of the residual fiber lignin was accompanied by extensive removal of chromophoric

groups. It appeared that the chemical structure of lignin remaining in the

fibers after the OQP sequence was mainly unaffected by the treatment. The oxidation

resulted mainly in an increase in the number of hydrophilic groups, but the

lignin remained phenolic to a certain extent and the aromatic structure was preserved

[25].

858 7Pulp Bleaching

A new mechanism for the heterogeneous alkaline peroxide brightening reactions

of mechanical pulps consists of four key kinetic steps: adsorption of H2O2

and hydroxide to the pulp fiber walls; a chromophore-removing chemical reaction

on the fiber wall; desorption of “light” organic products formed from the fiber

wall; and oxidation chain reduction of the cleaved organic substances. The most

important step here is the surface reaction, rather than reactions occurring in the

liquid phase. In general, removal of the cleaved organic substances from the fiber

wall is not anticipated to occur completely during the brightening reaction operation

stage [26].

As shown, the main reaction mode of HO2

– is nucleophilic addition to enone

and other carbonyl structures, removing chromophoric groups by the destruction

of conjugated systems. Through addition of the hydroperoxide anion, certain peroxide

(anion) structures may be formed which can subsequently react in a way

similar to that of the peroxide (anion) structures arising from the addition of

superoxide anion radicals to substrate radicals; this gives rise to the formation of

C–C cleavage products [27–30].

Due to the fact that the number of enone and other carbonyl structures in lignin

and residual lignin is usually low, the extent of degradation during bleaching with

pure H2O2 also remains low. Therefore, the main part of this bleaching step is

chromophore removal and lignin retention. Due to the fact that the number of

enone and other carbonyl structures in lignin and residual lignin is usually low,

and the extent of degradation during bleaching with pure hydrogen peroxide

remains low too. Therefore, the main part of this bleaching step is chromophore

removing and lignin retaining. However, this needs to be put into context with

two facts: a) most peroxide stages follow other bleaching steps where enone structures

are formed, and b) at high temperature extensive delignification can occur

[aa, bb].

The hydroxyl radical is considered to be responsible for the small degree of lignin

degradation observed during H2O2 bleaching. This can be interpreted as the

chemical reactions of the hydroxyl radicals during oxygen bleaching (see Section

7.3.2.4, Chemistry of oxygen delignification). The occurrence of hydroxyl radicals

may possibly have a distinct beneficial effect that may be ascribed to the cleavage

of cross-links in the rigid lignin matrix, which will in turn facilitate the penetration

of bleaching reagent(s) [31] and thereby improve the bleaching result. This

interpretation is in accordance with results from studies where metal ions were

removed carefully from either the pulp [32,33] or from wood shavings before kraft

cooking [34], or were complexed with chelants [25,35–38], and increased the

brightness gain [33].

7.6.4.3 Carbohydrates

The hydroxyl radical – but not the hydroperoxy anion – is capable of degrading

cellulose directly. Hydroxyl radicals, which are known to degrade carbohydrates

[39], have been generated photochemically from H2O2 in aqueous base, showing

that glycosidic linkages in methyl-b-d-glucoside and methyl-b-cellobioside cleave

7.6 Hydrogen Peroxide Bleaching 859

directly [40,41]. Evidence has been found for responsibility of the hydroxyl radicals

in the degradation of glycosidic linkages in 1,5-anhydrocellobitol and 2-methoxytetrahydropyran

by substitution reactions displacing 1-deoxyglucose, d-glucose, tetrahydropyran-

2-ol, and methanol [42]. Once the glycosidic linkages are broken,

the reducing carbohydrates undergo a series of reactions forming aldonic acids

and lower order aldoses, in much the same manner as was described previously

[40,41]. Under these same conditions, hydroxyl radicals cause a substantial degradation

of cellulose, as evidenced by a loss in viscosity [42].

Peroxides can degrade cellulose in the absence of stabilizing agents, as may also

decolorize it and remove stains. Both free radicals and hydroperoxy anions have

been suggested as the intermediates in the reactions occurring between cellulosic

products and H2O2 [43]. The oxidation of cellulose by H2O2 and the functional

groups formed revealed that the relationships between the functional groups, degradation

and stability of the celluloses enable the aging and storage behavior of

the polymer to be predicted. The “active” carbonyls are responsible for the peeling

reaction and formation of the yellow chromophore in alkaline solutions [44].

Experiments carried out on fully bleached pulp and viscose pulp showed clearly

that colored materials were formed from carbohydrates when they were submitted

to alkaline cooking conditions. However, these chromophores could be only partly

removed by H2O2 [45].

7.6.5

Process Parameters

Hans-Ullrich Sьss

7.6.5.1 Metals Management

Although transition metals cause the decomposition of H2O2, a controlled decomposition

with the well-defined generation of radicals would be desirable from the

point of improving delignification. However, to date, no such selective generation

has been described. A manganese containing complex [46] has been described as

catalyst for peroxide bleaching. Unfortunately, synthesis of this manganese complex

is rather difficult, therefore its industrial use would be far too costly. Typically,

the radicals produced by metal-catalyzed decomposition are unselective, and fiber

damage dominates as a result of cellulose depolymerization. In consequence, metal

impurities must be removed from the pulp before any subsequent peroxide

treatment [36,46,47]. The amounts of transition metals present in pulp differ

widely, as levels depend on the wood species and the soil on which the wood was

grown. Normally, manganese and iron are the dominant metals, and others such

as copper and cobalt are present only in trace amounts (around 1ppm). In sulfite

pulping, the removal of metal is straightforward since, under the acidic and reducing

conditions of the pulping process, the metals become water-soluble and are

easily removed during brownstock washing.

In kraft pulping, the transition metal ions become insoluble as they are reduced

to a low state of oxidation and precipitate as sulfides. The sulfides are very insolu-

860 7Pulp Bleaching

ble under alkaline and neutral conditions and cannot be removed by washing.

During oxygen delignification, the metals may be raised to a higher state of oxidation,

although the resulting hydroxides are still insoluble under the conditions of

oxygen stage washing. However, they become water-soluble under mild to strong

acidic conditions. In conventional bleaching processes, the transition metals are

removed during the acidic bleaching stages. Since H2O2 typically is applied in ECF

bleaching only after the first D stage, the metal profile normally is already sufficiently

low, and no specific measures for metal removal are required. The effect of

pH value on the elimination of iron and manganese from a softwood kraft pulp is

shown graphically in Fig. 7.118. Compared with iron, the removal of manganese

is clearly much easier. Strong acidic conditions are required to reduce the quantity

of iron, which is very likely bound to lignin or lignin–carbohydrate structures.

The iron is therefore not directly available for to decompose H2O2, and consequently

traces remaining in the pulp after chelation do not have a negative effect

on the bleaching process.

7 6 5 4 3 2

0

20

40

60

80

100

Initial

Fe Mn

Metals [ppm]

pH value

Fig. 7.118 Removal of iron and manganese from softwood

kraft pulp with increasing acidity. All trials conducted at 3%

consistency, 60 °C, 0.5 h with H2SO4 for acidification.

The removal of metals is far more important in TCF bleaching, because H2O2 is

applied early in the sequence, and at much higher charges. Since strongly acidic

conditions have the disadvantage of removing not only metals such as manganese

but also magnesium (which protects against loss of viscosity), metals removal at

the mill scale is typically carried out at moderate pH with chelants such as diethylene

triamino penta-acetate (DTPA). The impact of increasing amounts of chelant

is shown in Fig. 7.119, where DTPA addition maintains a high level of magnesium.

Typically, a chelation stage (Q) is operated at medium consistency, a temperature

between 50 °C and 70 °C, a pH of about 6, and a retention time of about 1h.

As mentioned, it can be assumed that any remaining traces of metals are tightly

7.6 Hydrogen Peroxide Bleaching 861

0 0.25 0.5 1

DTPA (%)

0

20

40

60

80

100

metals amount (ppm)

Mn Fe

Fig. 7.119 Removal of iron and manganese from softwood

kraft pulp with diethylene triamino penta-acetate (DTPA) at

pH 6. Trials were conducted at 3% consistency, 50 °C, 0.5 h,

with H2SO4 for acidification.

bound to the pulp; hence, it is impossible to provide a “threshold” no-effect level

for a metal residual. Indeed, it is more important to have an effective washing system

in place that guarantees the removal of the highly soluble metals portion.

In mechanical pulp bleaching, or in the bleaching of annual plants (e.g., bagasse),

the use of phosphonates can be advantageous. The phosphonate which is homologous

to DTPA – diethylene triamine penta methylene phosphonic acid (DTMPA) –

forms complexes with a higher chelation constant, and is therefore more effective in

removing metals that are bound tightly to cellulose or lignin complexes.

7.6.5.2 Alkaline Decomposition of H2O2

The active species in H2O2 bleaching is the perhydroxyl anion. This is generated

under alkaline conditions, by the addition of caustic soda. Because H2O2 decomposes

at high pH [see Eq. (112)], a very high pH-value in bleaching is detrimental.

The oxidation process generates acidic compounds, and this causes a decrease of

the pH during the bleaching procedure. Typically, in peroxide bleaching the initial

pH is between 10 to 11, whilst the end pH is still above 8.5. In sulfite pulp bleaching,

MgO can be used during the peroxide and oxygen stages to allow recycling of

effluent into the recovery of (magnesium sulfite) pulping liquor. The bleaching

efficiency is lower compared with caustic soda, due mainly to limited solubility

and lower pH. In addition, less hemicellulose is extracted from the pulp, which

may be advantageous in mechanical pulp bleaching. There in addition, sodium

silicate is added to the bleaching process, as silicate buffers the pH value and stabilizes

peroxide consumption.

Other compounds producing an alkaline pH are technically not applied, mainly

because of cost considerations. As an alternative to sodium silicate, the use of

sodium carbonate is limited to moderate temperatures, since above about 50 °C

862 7Pulp Bleaching

0 10 20 30 40 50 60

0

1x103

2x103

3x103

4x103

5x103

Temperature [°C]:

RT 30 40 50 60 70

hydrogen peroxide conc [ppm]

Time [min]

Fig. 7.120 Stability of bicarbonate-buffered peroxide solutions

in distilled water at different temperature, constant charge of

analytical grade NaHCO3 (20 g L–1).

the carbonate causes peroxide decomposition [48]. Solutions containing higher

levels of carbonate ions can be used in bleaching processes only after the addition

of magnesium sulfate, which precipitates the carbonate ions as very insoluble

MgCO3, or magnesium hydroxide carbonate, 4 MgCO3.Mg(OH)2. The instability

of peroxide solutions in deionized water in the presence of carbonate ions is

shown in Fig. 7.120. At 70 °C, an amount of 5000 ppm of H2O2 decomposes

almost completely within about 1h. This decomposition of H2O2 in the presence

of carbonate ions has been described previously [49], though no satisfactory explanation

was provided for any negative effects. The effects could not be explained by

speculation about traces of metals and “impurities”; neither was the link recognized

to the precipitation of carbonate by magnesium ions.

7.6.5.3 Thermal Stability of H2O2 and Bleaching Yield

The temperature in bleaching can be varied within a wide range. Logically, a lower

temperature results in a slow bleaching reaction, but this can be compensated for

by extending the retention time. Peroxide bleaches at ambient temperature, and

this allows an application in steep bleaching with a time range of days. Mechanical

pulp and sulfite pulp is bleached on an industrial scale under such conditions,

but these are rare exemptions. Typically, bleaching with H2O2 employs a temperature

range between 70 °C and 90 °C. The huge amounts of pulp handled in continuous

processes does not allow long residence times, or the bleaching towers

would need to be very large. Temperature and time are interrelated. The trend to

use narrow water loops with a high level of internal recycling, leads to high tem-

7.6 Hydrogen Peroxide Bleaching 863

peratures within the loops. Pulping and refining processes are operated above 100 °C,

and today even screening and cleaning of the pulp is conducted at a temperature

close to the pulp’s boiling point. In mechanical pulp bleaching, the temperature

typically is above 70 °C, but in chemical pulp bleaching it can be as high as 90 °C.

Consequently the time required for bleaching becomes short. A very high temperature

(>95 °C) is critical because H2O2 decomposes thermally. An example of this

reaction at a concentration typical of a bleaching process is shown in Fig. 7.121.

0,0 0,5 1,0 1,5 2,0

0,01

0,1

Temperature [°C]

70 85 100

peroxide residual [g/L]

Time [h]

Fig. 7.121 Decomposition of diluted alkaline H2O2 in deionized

water at pH 10.5 with temperature and time. Starting

concentration 2.5 g L–1; pH adjustment with NaOH.

The normal residence time for a peroxide stage is about 1.5 h. Depending on

the temperature and the amount of H2O2 to be consumed, this time may be

shorter and/or extended to 2–3 h. Pressure and very high temperature were

recommended for the consumption of large amounts of H2O2 in ECF and TCF

bleaching [50,51]. However, pressure is required only in so far as it allows a

bleaching temperature above 100 °C. At a temperature below the boiling point of

water, an increased pressure has no impact on peroxide performance. On the

other hand, a very high temperature in peroxide bleaching has a negative impact

on pulp quality. The energy of activation for cleavage of the oxygen–oxygen bond

of H2O2 is rather low; therefore, the side reaction “thermal decomposition” or

homolytic cleavage increases strongly with temperature (see Fig. 7.120). The aftermath

of this bond cleavage is the formation of other radicals, which trigger cellulose

chain cleavage. Viscosity losses are also observed which, together with the

improved solubility of lower molecular-weight compounds present in the pulp at

high temperature and alkalinity, leads to yield losses [52,53]. Thus, extreme temperatures

should be avoided in peroxide bleaching.

864 7Pulp Bleaching

An example of the impact of high temperature in peroxide-supported extraction

stages is provided in Tab. 7.48. The aggressive conditions allow less chlorine dioxide

to be used, but the impact on viscosity and yield is pronounced. Consequently,

in ECF bleaching the mill practice is to keep the temperature level below 90 °C

during the peroxide stages. The exemption is TCF bleaching, where a very high

temperature and even pressure must be applied to compensate for the absence of

an effective delignification agent such as chlorine dioxide. In this situation, the

consequences of a lower yield and decreased pulp strength must be accepted.

Tab. 7.48 Impact of very high temperature in peroxide stages on

pulp yield, effluent load, and viscosity. eucalyptus kraft pulp,

bleached under standard (Eop 0.4% H2O2, P 0.2% H2O2) and

hot conditions (Eophot 0.5% H2O2, Phot 0.8% H2O2) to a

brightness of >89% ISO.

Sequence Total

active chlorine

[%]

Temperature

in Eop or P

[ °C]