P Paa-P

Fig. 7.137 TCF bleaching of softwood kraft pulp with and

without peracetic acid activation of a final P stage. Pulp prebleached

with OO-Q-OP; Paa stage at 85 °C, 1 h, 10% consistency;

final P stage with 2% H2O2, 1.4% NaOH, at 95 °C, 3h,

10% consistency.

hydrogen peroxide. After about 1h the remaining peroxo compound will be predominantly

hydrogen peroxide.

Sequences with peracetic acid activation use the Paa step ahead of the final P

stage. Thus, a TCF sequence could be OO-Q-OP-Paa/P or OO-Q-OP-Paa-P. Peracetic

acid was also recommended as a final treatment step to boost the brightness

of TCF pulp [2]. However, because of the type of reactions occurring with the

remaining lignin, brightness stability is affected negatively. There is an improvement

in brightness, but not in its stability. In addition, the high temperature required

makes application in a high-density storage tower difficult.

Figure 7.138 illustrates the impact of a Paa post-treatment of a TCF pulp with

0.5% Paa at 80 °C. The improvement in brightness by more than 2 points is significant.

Brightness stability, however, decreases. This suggests the formation of

potential chromophores with Paa oxidation. These chromophore precursors are

removed by treating pulp with alkaline peroxide. The change in pH changes the

oxidant form from an electrophile into a nucleophile and thus removes quinones,

it is likely that Paa generates phenolic intermediates. Peracid treatment is therefore

more advantageous if followed with a peroxide stage.

The moderate speed of reaction of peracids with lignin is accelerated in the

presence of chloride ions. Because of their high oxidation potential, peracids will

oxidize chloride to chlorine. This acceleration increases the effect [3], it is, however,

not environmentally sound because halogenated compounds are generated.

Higher levels of AOX in the effluent and OX in pulp result from a reaction of peracid

in the presence of chloride [4]. This contradicts the purpose of TCF bleaching.

Peracetic acid should not be applied when there is a high level of chloride in the

water loop.

882 7Pulp Bleaching

P P-Paa P-Paa/P

brightness (%ISO)

0.2

0.3

0.4

0.5

0.6

0.7

post color #

brightness post color #

Fig. 7.138 Impact of a post-treatment with peracetic acid on

brightness and brightness stability. Paa stage with 0.5% at

80 °C, 1 h 10% consistency, Paa/P at 80 °C with 0.5% Paa and

alkali treatment after 0.5 h with 0.5% NaOH and continuation

for 1 h. Stability analyzed after hot humid reversion (E.4P).

7.8

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