Chlorine Dioxide Bleaching of Oxygen-Delignified Kraft Pulps

The main advantage of chlorine dioxide bleaching is undoubtedly its high bleaching

efficiency and selectivity. In numerous studies it has been shown that the

selectivity of chlorine dioxide is largely independent of the kappa number and viscosity

entering the D0 stage when applied to an unbleached pulp. Due to environmental

restrictions for bleach plant effluents and lower overall pulp manufacturing

costs, kraft pulps – even hardwood kraft pulps – are increasingly pre-delignified

in an oxygen delignification stage. The reactive species in oxygen delignification,

however, compete with the same type of lignin structures as those in chlorine

dioxide bleaching [4]. Consequently, the residual lignin after an oxygen stage

is more resistant to chlorine dioxide as compared to the residual lignin of an

unbleached kraft pulp. Barroca et al. demonstrated that the more pronounced

7.4 Chlorine Dioxide Bleaching 759

0 4 6 8 10 12 14 16

D

: Initial Kappa 14.6 D

: Initial Kappa 11

O: Initial Kappa 14.6 OD

: Initital Kappa 9.3

Viscosity, ml/g

Kappa number

0,4 0,6 0,8 1,0

0,0

0,1

0,2

0,3

0,4

D

: Initial Kappa 14.6 D

: Initial Kappa 11

O: Initial Kappa 14.6 OD

: Initial Kappa 9.3

Chain Scissions

Normalized Kappa number, κ/κ

Fig. 7.64 Comparison of the selectivity of oxygen

delignification (O) and bleaching of both

an unbleached chlorine dioxide (D0) and oxygen

pre-delignified E. globulus kraft pulp (OD0)

(according to [5]). Unbleached kraft pulps:

Kappa numbers 14 and 11, viscosities

1427 mL g–1 and 1250 mL g–1, respectively.

D stage: 55 °C, 3% chlorine dioxide charge,

maximum reaction time, 30 min. O stage:

95 °C, 10 kg NaOH odt–1, 8 bar, maximum reaction

time, 120 min. (a) Viscosity–kappa number

relationship; (b) chain scissions versus

normalized kappa number.

760 7Pulp Bleaching

degradation of polysaccharides during an OD-sequence (means oxygen delignification

followed by chlorine dioxide treatment) cannot be attributed to the action of

oxygen alone [5]. The selectivity of chlorine dioxide bleaching using an oxygen

pre-delignified kraft pulp made from E. globulus decreases considerably with

increasing extent of delignification in the oxygen stage (Fig. 7.64a,b). At a level of 40%

delignification, the number of chain scission is doubled when chlorine dioxide is

applied to an oxygen pre-delignified hardwood kraft pulp as compared to an untreated

pulp of the same provenance. Additionally, chlorine dioxide reactions with oxidized

lignin structures promote chlorate formation, as previously presented [6].

Surprisingly, oxygen delignification behaves rather selectively in the initial

phase, which concludes that the best selectivity of an OD sequence can be

obtained if oxygen delignification is limited to a very short reaction time of about

5–10 min. However, using an E. eucalyptus kraft pulp, only 25% of lignin can be

removed during the first delignification phase. Therefore, the decision must be

made from case to case whether the additional investment costs for this short

delignification stage would be more profitable than the higher bleaching costs of

a conventional pre-delignification D0 stage.

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