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

Dithionite.

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

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.

Bleaching of Mechanical Pulp

6.3

Technology of Mechanical Pulp Bleaching

Reductive bleaching is the most simple option for bleaching, not only because

Just one chemical has to be added to the pulp, but also because the available low

Or medium consistency after screening and latency chest can be applied. Other

Than using an effective mixer and a tower or tube, no additional equipment is required.

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