Sequence Stage Chemical Chemical charge Kappa

No.

Bright-ness

[%ISO]

Viscosity

[mL g–1]

AOX

[kg odt–1]

[kg odt–1] OXE R OXE

D0EOPD1D2 Unbleached

D0

EOP

D1

D2

ClO2

H2O2

ClO2

ClO2

14.0

5.0

10.0

2.5

14.8

87.7

940 0.8

ZEOPD1D2 Unbleached

Z

EOP

D1

D2

O3

H2O2

ClO2

ClO2

6.0

5.0

10.0

2.5

14.8

87.5

820 0.2

(DZ)EOPD1D2 Unbleached

(D

Z)

EOP

D1

D2

ClO2

O3

H2O2

ClO2

ClO2

8.0

6.0

5.0

7.5

1.5

14.8

3.9 54.4

69.4

86.2

88.9

885 0.4

D0: 3% consistency,

EOP: 10% consistency, 70 °C, 60 min, 3 kg MgSO4*7H2O odt–

1, 0.2 MPa O2.

D1: 10% consistency, 75 °C, 90 min.

D2: 10 % consistency, 80 °C, 180 min, 2 kg NaOH odt–1.

Z: 40% consistency, pH 2.5.

(DZ): 3% consistency, first 8 kg ClO2 during 10 min, 50 °C,

then O3 is charged.

In other words, 1kg (mol) of ozone replaces 4.2 kg (2.8 mol) of active chlorine

which, in terms of OXE, represents almost the same oxidation potential (125 OXE

from ozone replaces 117 OXE from chlorine dioxide). The data in Tab. 7.43 also

indicate that a complete substitution of ozone for chlorine dioxide is even more

efficient in replacing chlorine dioxide. There, 1kg (mol) of ozone substitutes

6.1kg (4.2 mol) of active chlorine, which exceeds the (nominal) oxidation power

of chlorine dioxide (125 OXE from ozone replaces 173 OXE from chlorine dioxide).

However, the major advantage of a (DZ) treatment over a pure Z-stage is certainly

the better preservation of pulp viscosity, and this is also reflected in better

strength properties. The full replacement of the D0 stage by a Z stage is slightly

more efficient in the case of a hardwood than for a softwood kraft pulp. In the

latter case, 1kg (mol) of ozone substitutes 3.7 kg (2.5 mol) of active chlorine.

Hardwood kraft pulp is certainly more susceptible to ozone bleaching due to the

presence of HexA as part of the kappa number. The AOX formation is linearly

7.5 Ozone Delignification 847

related to the chlorine dioxide charge. The lower AOX level in the case of the (DZ)

treatment is related to the lower chlorine dioxide charge, but not to a chemical

breakdown during the subsequent ozone treatment.

The optimum placement of ozone within a TCF-sequence for full brightness

(+88% ISO) and maximum strength properties remains an unresolved question

and thus the focus of many investigations.

It has been reported that the efficiency of a TCF bleaching sequence was

improved significantly when an ozone stage was arranged between two hydrogen

peroxide stages [129]. This could be shown in a direct comparison of the two

sequences OQP1ZP2 and OZQP1P2, where identical reaction conditions were

applied (the charge of OXE is equal for both sequences). It is interesting to note

that the brightness level prior to the last bleaching stage P2 is almost equal for

both sequences. However, the brightness gain in the second peroxide stage is 5%

ISO points higher when it succeeded an ozone stage rather than a hydrogen peroxide

stage, as shown in Tab. 7.44.

Дата добавления: 2015-10-21