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6.2 Bleaching with Hydrogen Peroxide

When bleaching aspen pulp, the impact on fiber volume of using Mg(OH)2 is

pronounced. Conventional bleaching to very high brightness (>85% ISO) is

accompanied by a decrease in the specific volume from >2 cm3 g–1 to 1.4 cm3 g–1,

or even less. The substitution allows the specific volume (also labeled as “bulk”) to

be maintained. In a set-up using the Mg(OH)2 stage as the main (high consistency)

stage and an additional (medium-consistency) stage with caustic soda as a

brightness adjustment step, it is possible to balance the parameters of bulk and

tensile strength to all levels in between (Fig. 6.8). The more moderate response in

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-

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6 Bleaching of Mechanical Pulp

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

77

78

79

80

81

82

Potential savings

Single stage Two stages

Brightness [% ISO]

H

2

O

2

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

0

4

8

12

16

20

Brightness gain [points]

P-Stage

H2O2 [%]

Y-Stage

2 3 4

0

4

8

12

16

20

H2O2 [%]

1st stage

2nd stage 2nd stage

1st 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

excess by bisulfite, Y-P sequence with refiner application of

dithionite.

1132

6.2 Bleaching with Hydrogen Peroxide

The brightness stability of mechanical pulp is much lower compared with fully

bleached chemical pulp. The reason for this is the high level of lignin remaining

in the fiber. Following bleaching, the structural elements which will re-generate

chromophores easily are mostly eliminated, although the presence of phenols

allows sufficient oxidation processes to yield a low brightness stability [35]. Post

color numbers after heat-or light- induced reversion are significantly higher in

comparison to chemical pulp. This is understandable in light of the large quantities

of phenols and phenol ethers in the remaining lignin. The light-induced

brightness reversion of mechanical pulp can be significant. However, whilst the

reversion of a newspaper in the summer sun might become apparent after only

an hour, this is of limited practical impact. Paper typically is not excessively

exposed to light, and its reversion in the dark is more important. Accelerated

aging in dry or humid tests can describe the stability of a brightness gain. The

response of a TMP to peroxide and P-Y bleaching and to accelerated aging using

hot and humid conditions is compared in Fig. 6.11. Brightness losses described as

points of brightness are relatively constant for the whole range tested. Losses with

humid reversion (100 °C, 100% humidity, 2 h) are around three points, with a

clear tendency for lower losses the higher the brightness. This results in a decreasing

post color number. The results confirm the experience with chemical pulp,

that the greater the removal of chromophores, the better the resulting stability.

The reductive post-treatment has post color numbers that are approximately

equivalent to the results after the P stage; thus, the brightness gained is not easily

lost.

0+1 2+1 3+1 4+1 5+1

62

64

66

68

70

72

74

76

78

ISO-Brightness [%]

P P-Y

H2O2 + Na2S2O4 [%]

Y

Dithionite 2 3 4 5

Peroxide amount [%]

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Post color number

Fig. 6.11 Bleaching and aging of spruce TMP with Y, P, or P-Y

bleaching. Initial brightness 55.5% ISO, peroxide bleaching

with silicate stabilization, destruction of the peroxide excess

with bisulfite, constant input of 1% dithionite in Y.

1133