Introduction

Among the oxygen-based bleaching chemicals, ozone is the most powerful oxidizing

agent, reacting readily with almost any organic material. The good delignifying

and brightening properties make ozone an attractive candidate to replace chlorine-

based bleaching agents. The use of ozone as a bleaching agent results in an

effluent which is free from organochlorine compounds and can be completely

recirculated to the chemical recovery system. Thus, ozone bleaching may be a prerequisite

for a closed-loop bleaching process. However, there are some difficulties

concerning the application of ozone bleaching in industrial practice. First, ozone

is an unstable gas which must be produced on site, most commonly by passing

oxygen gas through an electrical discharge where some of the oxygen molecules

are dissociated into oxygen atoms. In turn, oxygen atoms unite with oxygen molecules

to form ozone. Ozone generation technology in the early stages could produce

only 2–4% ozone by weight in an oxygen carrier gas. Later developments in

ozone generation technology could produce 5% by weight. In the early 1990s, concentration

of ozone could be raised to 8–12% by weight with power efficiency.

Recent advances in ozone generation which enable ozone concentrations up to

16% by weight, as well as the lowering of oxygen cost by means of on-site production,

have established ozone as a highly competitive bleaching chemical. The

ozone concentration can be further increased by compressing the gas mixture;

this improves the mass transfer from the gas into the liquid phase, which is a prerequisite

for an efficient bleaching process. Second, the high oxidation potential

of ozone makes it also prone to depolymerize and to degrade pulp polysaccharides.

In fact, its delignification selectivity is significantly lower than that of chlorine

dioxide. The prevalent view attributes this lack of selectivity to the generation

of highly reactive and nonselective hydroxyl radicals during the bleaching process.

The formation of hydroxyl radicals is usually ascribed to ozone self-decomposition

7.5 Ozone Delignification 777

in an aqueous system, to ozone decomposition catalyzed by transition metal ions,

and mostly to reactions between ozone and lignin structures, preferably containing

phenolic hydroxyls. Based on a huge research effort within the past decade,

the performance of ozone bleaching has been significantly improved with respect

to both selectivity and production costs, making ozone a competitive bleaching

agent. However, it has not yet been possible to increase the selectivity of ozone to

the same level exhibited by chlorine dioxide. This is a severe drawback for the production

of pulps where the high molecular weight of cellulose is a prerequisite to

attain the desired properties (paper-grade pulp: high-strength properties; dissolving-

grade pulp: high solution viscosity). Special emphasis will be given in future

research work to further improve the efficiency and selectivity of ozone bleaching.

Although the first implementation of ozone on industrial scale was until 1990,

when the first installation of an ozone bleach plant came on stream in Lenzing,

ozone has long been known as an efficient bleaching agent.

The reaction of ozone with textile fibers such as cotton and linen was studied as

early as 1868 [1]. In 1889, a method for bleaching “fibrous substances”, including

those used in the making of paper, with a mixture of chlorine and ozone gases

was patented by Brin and Brin [2]. Cunningham and Doree reported in 1912 that

ozone would preferably attack the lignin part in jute, but cellulose was also

affected [3]. In 1934, Campbell and Rolleston patented a process for bleaching

pulp by sequential treatment with chlorine and ozone [4]. Since the studies of Brabender

in 1949, in which he investigated some of the variables involved in ozonation

and patented a high-consistency ozone bleaching process, many reports and

patents on ozone bleaching have been published [5]. The breakthrough of ozone

bleaching was the invention and development of a technology to compress ozone

gas, and this is the prerequisite to apply ozone in medium-consistency technology.

Since the first industrial installation of an ozone plant in 1990, more than 25 pulp

mills with an annual production of about 8 million tons of pulp have implemented

ozone bleaching on industrial scale (see Tab. 7.39).

7.5.2

Physical Properties of Ozone

Ozone (O3) is an allotropic form of oxygen. At ambient conditions, it is a pale blue

gas (= 2.1415 g L–1 at 0 °C and 101.3 kPa). It condenses into an indigo blue liquid

(–112 °C) and freezes to a deep blue-violet solid (–195.8 °C). Ozone has a bent

structure of C2v symmetry with an apex angle of 116°49′and equal oxygen–oxygen

bond distances which are more closer to that of molecular oxygen as compared to

that of hydrogen peroxide. Hence, the bonds in ozone have considerable doublebond

character [6]. Data obtained from the microwave spectrum of the ozone molecule

have shown it not to be significantly paramagnetic [7]. Thus, the ozone molecule

can be pictured as a resonance hybrid consisting of four mesomeric structures,

as shown in Fig. 7.77.

778 7Pulp Bleaching

+O

O

O- O

+

O

O- -O

+

O

O -O

O

O+

1 2 3 4

Fig. 7.77 Resonance structures of ozone.

The contributions of forms 1 and 4, which have a positively charged terminal

oxygen with only six electrons, have been used to explain the electrophilic character

of ozone. As such, ozone falls into a moderately large class of 1,3-dipolar compounds

and will in certain reactions follow mechanisms typical of this class as a

whole [8]. In a simple molecular orbital representation of the ozone molecule,

each of the oxygen atoms is considered to be an sp2 hybrid and thus overlap of the

p-orbitals provides a molecular orbital containing four p electrons. The UV spectrum

of ozone shows an absorption maximum in 0.01M HClO4 at 260 nm with an

extinction coefficient of 2930 L.M–1.cm–1 [9]. In acid aqueous solution, the oxidizing

power is exceeded only by fluorine, atomic oxygen, OH radicals, and a few other

species [6]. The oxidation potential in aqueous solution is expressed by Eq. (83):

O3 _ 2H_ _ 2e_ _ O2 _ H2O_ E0 _ 2_07eV _83_

Ozone is thermodynamically unstable, and 1mol decomposes exothermically to

1.5 mol of molecular oxygen.

The solubility of ozone is an important criterion for ozone bleaching. The solubility

of ozone in equilibrium with its partial pressure is usually defined by

Henry’s law according to the following expression:

xO3 _

PO3

kH _84_

where xO3 is the dissolved ozone molar fraction (mol mol–1), PO3 is the ozone partial

pressure (kPa), and kHis Henry’s law constant (kPa mol fraction–1).

The ozone molar fraction can be transformed to a concentration of ozone, cO3

(in mol L–1 or mg L–1) by multiplying the molar fraction by 55.51or by 2.664 × 106,

respectively. Ozone solubility is influenced by several factors, such as temperature,

pH, ionic strength and dissolved matter. Henry’s law constant, kH, depends on the

temperature, T, according to Eq. (85):

dLnkH

d_1_T__ _

DH

R

kH _ k0

H _ Exp _

DH

R

1

T _

1

T0 __ _85_

where R is the gas constant and DH is the heat of solution of the gas. The parameters

k0

H and T0 refer to kH and T at standard conditions. Equations (84) and (85)

show that an increase in temperature is connected with a decrease in the dissolved

7.5 Ozone Delignification 779

780 7Pulp Bleaching

ozone concentration. The reduced ozone solubility at higher temperature can be

explained by a drop in the liquid phase driving force, and by a higher decomposition

rate. The pH is the predominant parameter which determines the stability of

ozone in aqueous solution (see Section 7.5.4), and hence also its solubility in

water. It is agreed that the dissolved ozone concentration increases with decreasing

pH. This is one of the important reasons why ozone bleaching is conducted

under acidic conditions, preferably in the pH range of 2–3. Quederni et al. have

determined the apparent Henry’s law constants for ozone solubility in water as a

function of temperature at pH 2 and pH 7 [10]. The relationship between kH and

the temperature for these two different pH levels is expressed in Eq. (86):

kH _ 101_3 _ Exp 20_7 _

3547

T __pH _ 7_

kH _ 101_3 _ Exp 18_1 _

2876

T __pH _ 2_

_86_

The solubility of ozone in water at 1atm, pO3 = 101.3 kPa and 0 °C calculates to

(101.3/1.966 × 105) × 2.664 × 106 = 1 . 37 g L–1 at pH 2, and to (101.3/

2.270 × 105) × 2.664 × 106 = 1.18 g L–1 at pH 7. Figure 7.78 illustrates the course of

the equilibrium dissolved ozone concentration as a function of temperature in the

range of 15 to 50 °C for the two pH levels, considering typical conditions for medium

consistency ozone bleaching:

_ Generated ozone concentration in oxygen gas: 200 gm–3 = 9.3 Vol%

_ Total pressure in the mixer: 8 bar = 0.81MP a

_ Ozone partial pressure, pO3: 0.093 × 810.4 = 75.4 kPa

10 20 30 40 50

100

200

300

400

500

600

p

O3

= 75.4 kPa

pH = 7 pH = 2

Dissolved O

3

conc. [mg/l]

Temperature [ºC]

Fig. 7.78 Influence of temperature and pH on the dissolved

ozone concentration in water assuming a partial pressure,

pO3, of 75.4 kPa (according to results determined by Quederni

et al. [10]). The ionic strength is kept constant at 0.13mol L–1.

The ratio between the ozone solubility at pH 2 and pH 7 further increases with

rising temperature. The dependency of the dissolved ozone concentration on pH

was even more pronounced, according to results obtained by Sotelo et al. [11].

These authors found an increase by a factor of 1.6 (3.1) when comparing ozone

solubilities at pH 2.5 and pH 7.0 (pH 9.0) at 10 °C and an ionic strength of 0.15 M.

As already mentioned, gas absorption is also dependent on the ionic strength.

Generally, the dissolved gas concentration decreases as ionic strength increases.

The effect of ionic strength on the solubility of ozone is most pronounced in the

presence of phosphates, chloride or carbonate ions, whereas the addition of sulfate

ions exerts practically no change in solubility. According to Sotelo et al., the

dissolved ozone concentration is decreased by half when increasing the ionic

strength from 0.04 mol L–1 to 0.49 mol L–1 in the presence of sodium chloride (pH

5.94 and 1.1 kPa →4.8 mg L–1 versus 2.4 mg L–1) [11].

In more general terms, both the influence of temperature and ionic strength is

described by Eq. (5) [12]:

kH _ Exp _

2297

T _ 2_659 _ l _

688 _ l

T _ 16_808_ _87_

where kH, Henry’s constant is kPa.L.mol–1, T, temperature in Kelvin, and l is the

molar ionic strength. The temperature-dependence of the dissolved ozone concentration

is more pronounced using Eq. (86) than with Eq. (87), with an almost perfect

correspondence at temperature higher than 35 °C (Fig. 7.79).

0 10 20 30 40 50

0

200

400

600

800

1000

1200

1400

p

O3

= 101.3 kPa

Oyama: = 0.13 MQuederni et al.: = 0.13 M

= 1.00 M

Dissolved O

3

-conc [mg/l]

Temperature [ºC]

Fig. 7.79 Comparison of calculated dissolved ozone concentration

as a function of temperature using Henry’s constants

from different literatures sources: Quederni et al. [10] versus

Oyama [13]. The influence of ionic strength was assessed

using Eq. (87).

7.5 Ozone Delignification 781

7.5.3

Ozone Generation

Ozone is produced at the site of use because it is unstable and cannot be stored.

The ozone-generating system is selected according to the requirements on site,

including the ozone bleaching technology (medium- or high-consistency), the

source of oxygen (cryogenic or adsorption), the temperature of cooling water, and

the possibilities to recycle the vent gas for oxygen delignification. Figure 7.80 illustrates

the principal elements of an ozone bleaching system, including the oxygen

source, the ozone-generating system, the ozone delivery system with an ozone

compressor in the case of medium-consistency ozone bleaching technology, the

mixer or reactor, the off-gas destruction system and the vent gas recovery and

recycle loops.

Oxygen

Source

Ozone

generator

Ozone

compressor

Mixer /

Reactor

Ozone

Destruction

Cooling

water

O2

O2/O3

Pulp O3 treated Pulp

Vent gas

O2 gas to recycle

or reuse

Fig. 7.80 Principal course of ozone in a pulp

bleaching system (according to [14]).

Ozone is produced from oxygen-containing gases in ozone generators by means

of silent electrical discharge in the so-called “corona discharge process”. To date,

in bleaching operations only oxygen gas is used to achieve a high ozone concentration

and to avoid the formation of reactive byproducts such as nitric acid. Oxygen

is passed through two electrodes which are separated from each other by a

dielectric and two discharge chambers (Fig. 7.81). When a high voltage is applied

between two concentrically arranged electrodes, and the voltage exceeds the ionization

potential of the dielectric material, then electrons flow across the gap and

782 7Pulp Bleaching

provide energy for the dissociation of oxygen molecules; these then combine with

oxygen molecules to form ozone. The key element of a corona discharge ozone

system is the dielectric. The electrical charge is diffused over this dielectric surface,

creating an electrical field where high-energy electrons bombard gas molecules

so that they are ionized and a light-emitting gaseous plasma is formed,

which is commonly referred to as a “corona”. Many different materials in a variety

of configurations are used for the dielectric, including scientific-grade glass (e.g.,

borosilicate) and nonglass materials such as silicone rubber. The quantity of

ozone produced is related to a number of factors, such as the voltage and frequency

of the alternating current applied to the corona discharge cells, the cooling

system, and the design of the ozone generator.

O2

O2/O3

Outer ground

electrode

Discharge

gap

HV electrode

Cooling

water

dielectric tube

Fig. 7.81 Schematic diagram of an ozone generation system.

The generally accepted technologies can be divided into three types: low-frequency

(50–100 Hz); medium-frequency (100–1000 Hz); and high-frequency

(1000+ Hz). Medium-frequency ozonators are now favored as they provide many

benefits over the older low-frequency technology. An example of this is a greater

ozone production with less electrode surface area, so that the equipment can be

smaller for a given ozone output, and the power consumption per kg ozone produced

is also reduced.

Since ozone generation by corona discharge is an exothermic physico-chemical

reaction, and ozone decomposition increases as the gas temperature and ozone

concentration increase, correct cooling is an important factor in generator design.

Moreover, oxygen entering the ozone generator must be very dry (minimum

65 °C), because the presence of moisture affects ozone production and leads to the

formation of nitric acid. Nitric acid is highly corrosive to critical internal parts of a

corona discharge generator, and this can lead to premature failure and a significant

increase in the frequency of maintenance. Besides the destruction of the

ozone generator itself, transition metal ions are released from the stainless steel

electrodes, and this can be very harmful to the pulp during the course of ozone

bleaching. Depending on the strength of the electric field, cooling and the design

of the ozone generator, ozone yields of up to 16% by weight (~240 g m–3) can be

7.5 Ozone Delignification 783

achieved in the production gas. The specific energy consumption for the production

of 1kg ozone is usually between 6 and 10 kWh, depending on the desired

concentration. The efficiency of medium-consistency ozone bleaching is limited

by a certain gas void fraction, Xg (according to Bennington, the upper operating

limit is reached at Xg = 0.13 [15], and according to industrial experience at Xg ~0.25

[16]). The gas void fraction is defined by Eq. (88):

Xg _

Vg

Vg _ VL

XR _

Vg

VL

Xg _

VR

1 _ VR

_ XR _

Xg

1 _ Xg

_88_

where:

Vg,T,P= Vg_T0_P0 _ 101_3 _ T

P _ 273_15 is the volumeof the gas fraction, with P in kPa and T in K;

VL= Prod

_con _ qsusp_

is the volume of the aqueous pulp suspension;

XR is the volume ratio;

Prod is the standardized pulp production (e.g., 1odt pulp);

qsusp = 1

con

1_53 _ _1 _ con_

qliquid _

is the density of the pulp suspension;

con is the pulp consistency, expressed as a fraction; and

qliquid ~1is the density of the liquid.

Both high-concentration ozone feed and compression of the feed gas are required

to ensure an efficient ozone consumption rate in medium-consistency

ozone bleaching. Compression is exclusively carried out isothermally by means of

liquid-ring compressors to avoid ozone destruction. The influence of ozone concentration

in the feed gas to the compressor on the ozone charge being efficiently

consumed in a medium-consistency mixer at a constant pressure of 8 bar at typical

industrial conditions (T = 50 °C, Xg,max = 0.25) is shown in Tab. 7.36.

The data in Tab. 7.36 indicate that the ozone charge in a medium-consistency

ozone mixer is limited to 3.2–4.2 kg odt–1. Clearly, the efficiency of medium-consistency

ozone bleaching also depends on the specific energy dissipation, e, and

on the retention time (see Section 7.5.5.2, Mixing). However, in the case of a modern

medium-consistency mixer the addition of higher ozone charges is connected

with decreasing amounts of ozone consumption rates (see Fig. 7.107).

784 7Pulp Bleaching

Table 7.36 Effect of ozone concentration in oxygen gas prior and

after compression to 0.8 MPa on the limit of ozone charge in a

medium consistency ozone mixer.

Ozone concentration in oxygen Xg

c Max. O3-charged

wt% Vol. % C at STa

[g Nm–3]

c at I NDb

[g m–3]

for 1 kg

[O3 odt–1]

[kg O3 odt–1]

5.0 3.4 72.6 491 0.17 1.4

6.0 4.1 87.4 591 0.15 1.7

6.8 4.7 100.0 676 0.13 1.9

7.0 4.8 102.3 692 0.13 1.9

8.0 5.5 117.3 793 0.12 2.2

9.0 6.2 132.5 896 0.10 2.4

10.0 6.9 147.7 999 0.09 2.7

10.2 7.0 150.0 1014 0.09 2.7

11.0 7.6 163.0 1102 0.09 2.9

12.0 8.3 178.5 1207 0.08 3.2

13.0 9.1 194.0 1312 0.07 3.4

13.4 9.3 200.0 1352 0.07 3.5

14.0 9.8 209.7 1418 0.07 3.7

15.0 10.5 225.4 1524 0.06 3.9

16.0 11.3 241.3 1632 0.06 4.2

16.5 11.7 250.0 1691 0.06 4.3

19.6 14.0 300.0 2029 0.05 5.1

a. ST = standard conditions: T0 = 273.15 K, P0 = 101.3 kPa.

b. IND = industrial conditions: T = 323.15 K, P = 810.6 kPa = 8 bar.

c. Xg = Vg/(Vg+VL) at 10% pulp consistency and IND.

d. Assuming an upper limit of Xg = 0.25 to obtain a reasonably

high ozone consumption rate.

7.5.4

Chemistry of Ozone Treatment

Manfred Schwanninger

Ozone treatment is a very effective way to remove residual lignin that remains

after pulping. The structure and reactivity of the residual lignin have already been

described (see Section 7.3.2.2). One of the major disadvantages of ozone as a

7.5 Ozone Delignification 785

bleaching agent is its moderate stability in aqueous solutions [1–4]. It has the tendency

to decompose in water, generating some very reactive, highly unselective

radical species [1–5]. Hydroxide ions are known to catalyze ozone decomposition

and to promote hydroxyl radical formation [1,3,4,6], whilst another drawback is

the undesired degradation of cellulose [7–23].

7.5.4.1 Ozone Decomposition

The pathways and kinetics of the decomposition of aqueous ozone are of interest

for a wide range of topics, not only for pulp bleaching, and have therefore been

studied intensively [1–5,24]. The chain mechanism of ozone decomposition

(Scheme 7.28) is based on the studies of Bühler et al. [5], while Staehelin et al. [1]

showed the decomposition of ozone and the formed intermediates (Scheme 7.28).

O2

-

HO2

O2

HO4

O3

OH

HO3

O3

-

1O2

O3

H2O

OH-

O2

H2O

H+

HO4

+

H2O2 + 2O3

HO3 H2O2 + O3 + O2

+

Termination

HO4

HO4

Scheme 7.28 Chain mechanism of ozone decomposition

according to Staehelin et al. [1].

The decomposition occurs by a radical chain mechanism, which in pure water

is initiated by the reaction between hydroxide ions (OH–) and ozone [Eq. (89). Superoxide

_O2

– then reacts with ozone rather selectively as part of a chain cycle [1].

O3_OH_→HO2 _ _ _O_2 _89_

The hydroxide-ion-catalyzed decay of ozone is expressed in the following general

rate equation (1):

d_O3

dt _ _k _ _O3a _ _OH_b _90_

The reaction order b with respect to hydroxide ion concentration varies from

0.36–1.0 and the reaction order a for ozone is reported to vary between 1.5–2.0 [aa,

bb]. Pan et al. have studied the decomposition rate of ozone in a pure aqueous

786 7Pulp Bleaching

7.5 Ozone Delignification 787

2 4 6 8

0

30

60

90

120

150

second order reaction rate

O

3

-concentration [mg/l]

Time [min]

Second Order Rate Constant

[mol-1*l*min-1]

pH

0 5 10 15 20 25 30

5

10

15

20

25

30

[O

3

] at pH 3 [O

3

] at pH 7

Fig. 7.82 Effect of pH on second order rate constant of ozone

decomposition and the course of ozone concentration in aqueous

solution at 25 °C according to Pan et al. [25].

system as a function of pH at 25 °C. The results revealed a rapid increase of the

second order rate constant from pH 4 to 7 as seen in Fig 7.82.

The concentration of dissolved ozone decreases rapidly at a neutral pH, while it

remains quite stable under acidic conditions (pH 3) within a time frame relevant

for industrial ozone bleaching applications.

The first propagation step of the chain reaction proposed by Weiss for the

decomposition of aqueous ozone can be described with the intermediates _O3

–

and HO3_ [5]. The elementary reactions of these species and their constants are

presented in Eq. (91) [5]:

O3__O_2 →_O_3 _1O2 k_ 1_6 _ 109M_1s_1

_O_3 _H_

ka

kb

HO3 _ ka _ 5_2 _ 1010M_1s_1 kb _ 3_7 _ 104M_1s_1

HO3 _→HO_ _ O2 k_ 1_1 _ 105M_1s1_1

_91_

In water, the decay of HO3_ is very rapid, with a half-life of about 6 ls. Since

_O2

– reacts rapidly with ozone, but relatively slowly with organic compounds, the

latter will interfere with ozone decomposition in aqueous solutions by scavenging

OH radicals rather than _O2

– [5]. The second propagation step and the reactions of

these species and their constants are presented in Eq. (92):

HO_ _ O3→HO4 _ k_ 2_0 _ 109M_1s_1

HO4 _→HO2 _ _ O2 k_ 2_8 _ 104M_1s_1

HO2 _ _ _O_2 _H_ pKa_ 4_8

_92_

The transient HO4_ might be a charge-transfer complex (HO_O3) [1]. The lifetime

of HO4_ was found to be much longer than its accumulation rate, and therefore

it acts as a carrier reservoir within the chain cycle. As a consequence, HO4_

is the important transient for chain termination reactions (Fig. 7.82). The termination

reactions shown in Eq. (93) are the dominating ones in the presence of

high ozone concentrations [1].

HO4 _ _ HO4 _→H2O2_2O3

HO4 _ _ HO3 _→H2O2_O3_O2 _93_

If, at low ozone concentrations, organic solutes are present, their dominant

effect will be to withdraw OH radicals; at high ozone concentrations this will similarly

occur with HO4_. Some organic materials are thereby able to regenerate _O2

–

in order to sustain the chain.

As the initial step of the ozone decomposition is the reaction between ozone

and the hydroxide anion [Eq. (89) and Scheme 7.28], strong pH dependence is

expected [6]. Gurol and Singer [4] investigated the kinetics of ozone decomposition

in the pH range from 2 to 10. They found that ozone decomposes rapidly at a pH

above about 6.5, but remains quite stable under acidic conditions; indeed, this

finding has been confirmed by several authors [6,25,26]. Hydrogen peroxide, also

used as a bleaching chemical, is incapable of initiating ozone decomposition;

however, its deprotonated form, the hydroperoxy anion (HO2

–) has such ability [6].

At pH < 12 and hydrogen peroxide concentrations >10–7 mol L–1, HO2

– has a

greater effect on the decomposition rate than the hydroxide anion (OH–) [3], and

this might be important for bleaching sequences. In the radical-type chain reaction

decomposition of ozone, inorganic and organic compounds can be divided

into three categories: (a) initiators; (b) promoters; and (c) inhibitors [6,24,27,28].

Initiators are substances that are capable of initiating the decomposition of ozone

to the superoxide anion radical [Eq. (89)], while promoters are radical converters

forming the superoxide anion radical from the hydroxyl radical (Scheme 7.29).

Inhibitors are substances that react with the hydroxyl radical without the formation

of superoxide anion radical, called radical scavengers, such as bicarbonate

and carbonate, leading to the corresponding radicals [24]. Some examples are given

in Tab. 7.37.

CH3OH

OH H2O

H2C

OH

H2C

OH

O2

OO

O2

-

B- BH

H2CO

Scheme 7.29 Reaction of methanol as a typical hydroxyl radical

to superoxide anion radical converter acting as a promoter [24].

788 7Pulp Bleaching

Tab. 7.37 Typical initiators, promoters, and inhibitors for

decomposition of ozone by radical-type chain reactions

[6, 24–28].