Consumed

[kg odt–1]

Bleaching

Sequence

Kappa

Number

Viscosity

[mL g–1]

DKappa/CS

0 34.0 1280

19 Z 17.6 920 12.5

19 ZE 12.6 905 15.3

19 ZEP 11.9 885 14.7

19 ZE0 9.5 850 14.2

The use of an E-stage following an ozone stage reduces the ozone charge by 25–

45% when bleaching to a certain kappa number target. Intermediate washing or

neutralization does not affect the extent of lignin removal during subsequent alkaline

extraction. However, neutralization directly after the ozone stage appears to

improve selectivity when followed by alkaline extraction.

7.5 Ozone Delignification 825

In the case of oxygen prebleaching, being the more realistic alternative, the saving

of ozone reaches almost 50% [82]. The viscosity values of the OZE-bleached

pulps correspond to those determined for the OZ-bleached pulps after reduction

with borohydride. Fiber strength (zero-span tensile index) is almost not impaired

by the E-stage (in relation to the Z-treated pulp), at least when using LC bleaching

technology. There are indications that more lignin is removed after LC and MC

ozone bleaching than after HC bleaching [90], but as yet this observation is not

understood.

7.5.6

Technology of Ozone Treatment

Andreas W. Krotscheck

Medium-Consistency Ozone Treatment

The process flowsheet of a typical medium-consistency ozone delignification system

is shown schematically in Fig. 7.99. MC pulp coming from the previous

bleaching stage falls into a standpipe after sulfuric acid has been added to adjust

the pH. The pump forwards the pulp suspension to a high-shear mixer which is

charged with compressed ozone/oxygen gas.

MC PUMP

HIGH-SHEAR

MIXER

BLOWTANK WASHING

O3

Pulp from

Preceding

Stage

H2SO4

Pulp to

Next stage

Offgas

Fig. 7.99 Process flowsheet of a typical medium-consistency

(MC) ozone delignification system.

It is of utmost importance that the ozone and pulp are mixed intensively,

because the predominant portion of the delignification occurs inside the mixer.

This is why the medium-consistency ozone system does not have a reactor comparable

to other bleaching applications. Instead, the mixing time is prolonged at

high power dissipation and, on occasion, a second high-shear mixer is installed

for that purpose. Additional time for the reactions to complete after the mixer is

usually provided by the pipe to the blowtank. This pipe may be increased slightly

in diameter to offer about 1min of retention time.

826 7Pulp Bleaching

The pressurized three-phase flow coming from the mixer expands into the blowtank,

where the pulp suspension is separated from the gas phase. The offgas is

cleaned of fibers in a scrubber and proceeds to the ozone destruction unit. The

pulp slurry is discharged from the blowtank either at low or medium consistency,

depending on the feed requirements of the subsequent equipment.

Washing after the ozone stage is often omitted, and the pulp is forwarded to the

subsequent stage at medium consistency. Otherwise, washing can be carried out

with single-stage washing equipment, for example with a single-stage Drum Displacer

™, a wash press, or a vacuum drum washer.

The material for the construction of wetted parts in an ozone stage is typically a

higher grade of austenitic stainless steel.

Further information regarding ozone delignification equipment including medium-

consistency pumps, mixers and blowtanks is provided in Section 7.2. Details

of pulp washing are provided in Chapter 5.

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