Pressurized Peroxide Bleaching

In TCF bleaching the brightness target of a final peroxide stage might require the

consumption of very large amounts of peroxide, and to achieve sufficient consumption

of H2O2 high temperature will be required. The application of pressure

is also recommended [50,51], with pressure applied ranging from 0.1 MPa to a

maximum of 0.5 MPa. These stages are frequently labeled as P(O) stage because

pressure is applied by oxygen gas addition. The positive effect was described as an

acceleration of an otherwise very slow brightening of the pulp. Later, pressurized

peroxide stages were also recommended for ECF sequences [56]. Several mills

have installed such equipment, although many typically operate without applying

pressure because bleaching is not improved by pressure or oxygen addition

[57,58]. This is in line with current knowledge of the reaction mechanism of alkaline

peroxide bleaching. The peroxide reactions are neither accelerated nor

improved by a moderate pressure increase, however, pressurized equipment is

more expensive than nonpressurized counterparts.

7.6.6

Technology of H2O2 Bleaching

Andreas W. Krotscheck

Atmospheric Peroxide Bleaching

The process flowsheet of a typical atmospheric peroxide bleaching system is

shown schematically in Fig. 7.122. Caustic soda is added to the MC pulp coming

from the previous bleaching stage, for example, to the repulper of a drum washer

or to the dilution conveyor after a wash press. The alkaline pulp falls into a standpipe

and is mixed with peroxide as it enters the MC pump.

The pump provides good mixing of the peroxide into the pulp suspension, and

a dedicated mixer is often not required. The pulp proceeds to an atmospheric

upflow reactor where the bleaching reaction takes place. Depending on the feed

requirements of the subsequent washing equipment, the pulp slurry is discharged

from the reactor either at low or medium consistency.

MC PUMP REACTOR WASHING

Pulp from

Preceding

Stage

Pulp to

Next stage

NaOH

H2O2

Fig. 7.122 Process flowsheet of a typical atmospheric peroxide bleaching system.

866 7Pulp Bleaching

Washing after a peroxide stage is usually carried out with single-stage washing

equipment, for example, with a wash press, a single-stage Drum Displacer™, an

atmospheric diffuser, or a vacuum drum washer.

The material of construction for wetted parts in a peroxide stage is typically a

higher grade of austenitic stainless steel.

Further information regarding atmospheric peroxide bleaching equipment,

including medium-consistency pumps and atmospheric upflow reactors, is provided

in Section 7.2. Pulp washing is detailed in Chapter 5.

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