Bleaching to the activation with Mg(OH)2 is compensated by a final hydrosulfite

treatment. Consequently, brightness is very high (>85% ISO) and will not effect

Any physical properties.

0 1 2 3

NaOH [%]

1.2

1.4

1.6

1.8

2.0

2.2

Bulk [cm./g]

Fig. 6.8 Decrease in specific volume of aspen TMP with input

Of caustic soda in a PMg(OH)2-PNaOH-Y sequence

(high consistency–medium consistency–low consistency).

Very high brightness can be achieved with two-stage peroxide bleaching, although

The equipment required is rather complicated (see Section 6.3). A twostage

Process can use the high excess of peroxide from the main bleaching stage

In a first step. This excess must be activated with an addition of caustic soda. The

Volume of liquid to be recycled depends on the dilution and dewatering conditions

Following the main bleaching step. Typically, volumes are so high that the first

Stage must be conducted at medium consistency. The potential savings of this

Approach (Fig. 6.9) are reasonable only for a very high brightness target, and this

Becomes apparent from the shape of the curves.

Some mills apply in-refiner-bleaching with sodium dithionite. Because of the

High temperature, this reaction is very rapid; sulfonation of the lignin occurs and

Typically the residual of sulfite detected in the pulp is extremely small. Therefore,

A peroxide bleaching stage can be added without any fear of activity losses. Y-P

Bleaching typically is applied in integrated mills producing paper with different

Brightness grades. A part of the reductive bleaching effect is lost by the oxidation

With peroxide, the reason for this being the re-formation of some conjugated

Structures with the oxidation. However, the total brightness gain in a Y-P treat-

Bleaching of Mechanical Pulp

2,0 2,5 3,0 3,5 4,0

Potential savings

Single stage Two stages

Brightness [% ISO]

H

O

to 2nd P-stage [%]

Fig. 6.9 Impact of single-stage (high consistency) or

two-stage (medium/high consistency) peroxide bleaching

Of a spruce TMP pulp. The second stage saves significant

Amounts of peroxide for very high brightness targets.

Ment using dithionite in the refiner and peroxide under high-consistency conditions

Delivers comparable results with the more conventional P-Y treatment. Post

Bleaching with dithionite requires destruction of the excess hydrogen peroxide

With sodium bisulfite or sulfur dioxide gas. These effects are explained graphically

In Fig. 6.10.

2 3 4

Brightness gain [points]

P-Stage

H2O2 [%]

Y-Stage

2 3 4

H2O2 [%]

St stage

Nd stage 2nd stage

St stage

Fig. 6.10 Comparison of P-Y and Y-P bleaching. Bleaching

of softwood TMP with constant 1% Na2S2O4 and variable

Amounts of peroxide. P-Y with destruction of peroxide

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