Sequences Preferably used for

D(EO)D SW-, HW-Kraft

D(EOP)D SW-, HW-Kraft

DEDED SW-, HW-Kraft

D(EO)DED SW-, HW-Kraft

D(EO)DD SW-, HW-Kraft

OD(EO)D SW-, HW-Kraft

OD(EOP)D SW-, HW-Kraft

ODEDED SW-, HW-Kraft

OD(EO)DED SW-, HW-Kraft

OD(EO)DD SW-, HW-Kraft

OQ(OP)(DQ)(PO) SW-, HW-Kraft

OD*(EO)D HW-Kraft

OA*D(EO)D HW-Kraft

O(AD)*(EO)D HW-Kraft

O(DQ)*(PO) HW-Kraft

* Denotes treatment with high

temperature and prolonged retention time.

756 7Pulp Bleaching

Bleaching of a softwood kraft pulp to full brightness (89+% ISO) is best accomplished

by a sequential treatment of two chlorine dioxide steps, D1 and D2,

because it is more efficient to split the chlorine dioxide charge into two stages.

The bleaching efficiency is further enhanced, when the D1 stage is followed by an

alkaline extraction stage, preferably including interstage washing. To attain a particular

brightness (e.g., 90% ISO), the minimum total amount of chlorine dioxide

consumption is obtained when 25% of it is applied in the D2 stage (Fig. 7.62). The

minimum is slightly shifted to a higher share of chlorine dioxide consumption in

the D2 stage while raising the final brightness target.

0 2 40 60 80 100

90 % ISO

ClO

consumed in D

and D

, kg/odt

Proportion of ClO

consumed in D

, % of total

Fig. 7.62 Effect of chlorine dioxide distribution between the

D1 and D2 stages of a CEDED sequence on the total chlorine

dioxide consumption in bleaching a softwood kraft pulp to

attain a final brightness of 90% ISO (according to [3]).

The task of chlorine dioxide bleaching in the final bleaching sequence is to

remove selectively the chromophores. The increase in brightness can be described

as occurring in two distinct steps: a very rapid and short increase, followed by a

much longer and slower period of reaction where the rate of decrease of the chromophores

almost approaches zero. The course of brightness increase is illustrated

in Fig. 7.63.

The brightening in the D2 stage responds to chlorine dioxide charge according

to the following type of expression:

BD 2 _ BD 1 _ D B max _ 1 _

_1 _ a _ ClO 2 ch _ b _ _

7.4 Chlorine Dioxide Bleaching 757

0 2 4 6 8 10 12 14

Brightness, % ISO

ClO

charge in the D

-stage, kg/odt

Fig. 7.63 Effect of D2 stage on the brightness of a softwood

kraft pulp bleached in a D0(EO)D1ED2 sequence.

where B D2 is the final brightness after D2 treatment, and B D1 is the initial brightness

before ClO2 addition. The term D B max is the brightness increase at infinite

ClO2 addition. The sum of B D1 and D B max represents the maximum achievable

brightness for the D2 stage (“brightness ceiling”); a and b are constants.

The typical bleaching conditions in D1 and D2 stages are listed in Tab. 7.34. The

amount of active chlorine required to attain full brightness is linearly related to

the kappa number of the pulp entering the D1 stage (after, e.g., the DE stage). The

chlorine dioxide charge can thus be estimated by measuring the E1 kappa number.

The brightness gain in the final D stage is different for hard- and softwood kraft

pulps. In most cases, the latter requires less chlorine dioxide to target full brightness.

Further information on the performance of chlorine dioxide in the final

bleaching stages is available elsewhere [3].

As mentioned previously, the pH profile has a major effect in determining the

efficiency of chlorine dioxide bleaching. During such bleaching, the pH is

decreased considerably during the first reaction phase due to the formation of

organic and hydrochloric acids. Hence, sodium hydroxide must be added to maintain

the optimal end pH in the range between 3 and 3.5 (D0). According to Reeve

and Rapson, approximately 0.6 kg NaOH odt–1 should be added for each kg

ClO2 odt–1 charged to the pulp to ensure an optimal end pH [3].

The final D stage is usually run with an excess amount of chlorine dioxide to

ensure a chlorine dioxide residual at the end of the stage. The chlorine dioxide

concentration in the filtrate of the pulp suspension leaving the final D stage is

maintained at between 10 and 50 ppm, and this ensures that both the brightening

758 7Pulp Bleaching

Tab. 7.34 Examples of typical chlorine dioxide bleaching

sequences.

Stage

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