Effect of Carry-Over

Closing the water cycle in totally chlorine (TCF) bleaching by countercurrent

washing results in an accumulation of dissolved organic material in the recycled

liquors. The concentration of the dissolved organic substances depends on the

washing efficiency of the washing equipment and the degree of closure of the

countercurrent water cycle. The waste material in the water associated with the

pulp affects the performance of ozone bleaching, depending on its origin. Ozone

delignification is very sensitive to dissolved material from preceding bleaching

stages (carry-over), but significantly less to its own (Z-stage filtrate or carry-back)

and subsequent bleaching stages (e.g., P-stage filtrate or carry-back). The high

reactivity of ozone towards carry-over can be explained by the low state of oxidation

of the waste material dissolved during oxygen delignification; this is roughly

characterized by the chemical oxygen demand (COD):dissolved oxygen concentration

(DOC) ratio being about 2.6 or slightly higher (for comparison, the COD:-

DOC ratio of native lignin is ~2.95 and of neutral carbohydrates is ~2.67). As the

ozone stage is normally placed after an oxygen delignification stage (O), the carryover

consists predominantly of an O-stage bleaching loss. The effect of carry-over

from oxygen delignification, as well as the carry-back from ozone and hydrogen

peroxide stages, on medium-consistency ozone bleaching of a eucalyptus prehydrolysis-

kraft (PHK) pulp was evaluated on the laboratory scale [28,41]. The load

of dissolved waste substances, expressed as COD, entering the ozone stage was

gradually increased to a value of 14.4 kg odt–1. Medium-consistency ozone bleaching

was carried out at two different levels of ozone charge, namely 2.0 and

4.0 kg odt–1. The ozone-bleached pulp was thoroughly washed and finally subjected

to standard hydrogen peroxide bleaching (10% consistency, 85 °C, 180 min,

5 kg H2O2 odt–1, 7 kg NaOH odt–1). As expected, the carry-over from oxygen stage

had a major impact on the final brightness (Fig. 7.95).

A carry-over of 14.4 kg COD odt–1 from the oxygen stage caused a decrease in

brightness from about 90% ISO to 85% ISO, while keeping the ozone charge constant

at 4 kg odt–1. The impact on brightness was the same as for an ozone charge

of only 2 kg odt–1 (Fig. 7.95). In agreement with other studies, there was no significant

effect of filtrate recirculated from the ozone stage itself on the ozone consumption

and on final pulp quality [69,70]. Small amounts of carry-back from the

P-stage even improved brightness, but this may be attributed to the residual

hydrogen peroxide (~ 1kg H2O2 odt–1) present in this solution. The amount of

additional ozone consumption per kg COD from the oxygen stage for a given

kappa number and brightness target after ozone and peroxide bleaching increased

with increasing extent of delignification in the Z-stage (Fig. 7.96).

816 7Pulp Bleaching

0 5 10 15

Carry over from O-Stage: 4 kg O

/odt; 2 kgO

/odt

Carry back from Z-Stage: 4 kg O

/odt; Carry back from P-Stage: 4 kg O

/odt

Brightness [%ISO]

COD in Filtrate [kg COD/odt]

Fig. 7.95 Effect of dissolved organic material

from O-, Z-, and P-stages on the brightness

after P-stage at a constant charge of ozone in

the preceding Z-stage (according to [28]).

Substrate: O-delignified eucalyptus-PHK pulp,

kappa number 3.0, viscosity 942 mL g–1.

Conditions in Z-stage: 9% consistency, 50 °C,

pH 2.0, 10 s mixing time, ozone charges:

2.0 and 4.0 kg odt–1.

0.6 0.9 1.2 1.5

0.0

0.1

0.2

0.3

carry over from O-Stage carry back from P-Stage

ΔO

/ COD [kg O

/kg COD]

Kappa number after Z

Fig. 7.96 Additional ozone consumption per kg COD derived

from dissolved organic material from O- and P-stages as a

function of the kappa number after Z-stage (according to

[28]). Substrate and conditions see Fig. 7.95.

7.5 Ozone Delignification 817

The data in Fig. 7.96 illustrate that an additional amount of 0.20–0.22 kg

ozone kg–1 COD from the oxygen filtrate is consumed to obtain a final brightness

of about 90% ISO; this corresponds to a kappa number after Z-stage of 0.8 to 1.0.

In other words, a COD carry-over of 10 kg odt–1 from the oxygen stage increases

the specific ozone consumption by 85% in the case of 85% ISO brightness

(0.17 kg O3 kg–1 COD. 10 kg COD odt–1 + 2.0 kg O3 odt–1 = 3.7 kgO3 odt–1), and by

55% in the case of 90% ISO brightness (0.22 kg O3 kg–1 COD. 10 kg

COD odt–1 + 4.0 kg O3 odt–1 = 6.2 kgO3 odt–1).

These values are in good agreement with those reported by other investigators

[70,71]. The effect of carry-back from the P-stage on additional ozone consumption

amounts to only about one-eighth of that of the carry-over from oxygen stage,

even when the effect of residual hydrogen peroxide is disregarded. The lack in

efficiency of ozone delignification in the presence of carry-over can be compensated

for by an additional ozone charge without impairing the selectivity in terms

of the viscosity–brightness relationship [41,70].

The effect of the carry-over/carry-back to the ozone stage is also strongly dependent

on pulp consistency. High-consistency ozone bleaching is much less affected by the

presence of dissolved organic material than medium-consistency ozone treatment

because of the significantly lower amount of water associated with the pulp.

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