Typical Conditions, Placement of Z in a Bleaching Stage

The placement of an ozone stage within a bleaching sequence must consider both

technological and chemical aspects. The low pH and high sensitivity towards

carry-over from the washing stage of an unbleached kraft pulp suggest that ozone

should not be used in a first delignification stage. Moreover, ozone degrades part

of the phenolic units and makes oxygen less reactive towards lignin in a ZOsequence.

In contrast to the observations of Lachenal et al. [122]. who found that a

single ozone stage (Z) behaves as selectively as an OZ-sequence, Brolin et al. [75],

Ragnar [106], as well as the results shown in Fig. 7.110, show that ozone bleaching

becomes more selective in terms of brightness increase per number of chain scissions

by lowering the incoming kappa number; this means that oxygen delignification

prior to the ozone stage is desirable for reasons of delignification selectivity.

In addition, OZ is favored over Z because of better process economy due to lower

chemical costs (lower ozone consumption and the possibility of recycling oxygen

from the Z-stage) and better possibilities to close the water cycle. The choice between

OZ and Z also depends on the applied ozone bleaching technology. In HC

ozone bleaching, a sufficient quantity of ozone can be reacted in order to achieve

the necessary extent of delignification in a single ozone stage, whereas ozonation

at medium-consistency is limited to a kappa number reduction of maximum 5–7

units (assuming a specific kappa number reduction of about 1unit per kg ozone

charged; see also Tab. 7.36) which in most cases is not enough to complete

delignification.

In a TCF-bleaching sequence consisting of O-, Z-, and P-stages, the use of

hydrogen peroxide (P) is essential to remove the chromophores by oxidizing the

carbonyl groups. As expected, the placement of a P-stage within such a sequence

affects the final bleached pulp properties. OZP- and OPZ-sequences show the

same delignification efficiency, while the latter appears to be more selective as

compared to OZP [122,123]. In a recent study, the effect of placing the Z-stage

prior to (ZP) and after (PZ) standard peroxide bleaching of an (E/O) pretreated

beech dissolving pulp was evaluated by charging different amounts of ozone while

all other reaction conditions were kept constant [123]. GPC measurements

revealed that cellulose degradation was more pronounced for ZP- than for PZtreated

pulps, while the latter had slightly lower brightness values (see Tab. 7.41

and Section 11.3.2.2.2). Figure 7.114 illustrates the course of cellulose degradation

in terms of weight (MW) and number (MN) average molecular weights.

7.5 Ozone Delignification 843

0 2 4 6

PZ-sequence: MW MN

ZP-sequence: MW MN

Molecular weight [kDa]

Ozone charge [kg/odt]

Fig. 7.114 Course of weight (MW) and number (MN) average

molecular weights of beech sulfite dissolving pulps as a function

of ozone charges with Z-stage prior to (ZP) and after Pstage

(PZ), applying identical conditions in each stage [123].

In contrast to the results obtained from Godsay and Pearce [99] and Berggren et

al. [121], the polydispersity index (PDI) – that is, the ratio of the weight average to

the number average molecular weights (MW/MN) – did not increase but rather

was slightly decreased, from about 6.8 in the untreated pulp to 5.5 in the most

severely degraded pulp. This may be attributed to the fact that the beech sulfite

dissolving pulps were subjected to significantly less ozone dosages (2–6 kg odt–1)

than those reported by either Godsay and Pearce (47.7–75.4 kg odt–1) or Berggren

et al. (1–35 kg odt–1).

The placement of Z within the TCF sequence also influences the shape of the

differential MWD. All samples displayed a shift of the MWD towards a lower molecular

weight range as degradation proceeded. The high molecular-weight cellulose

fraction of the pulp subjected to ZP treatment was considerably degraded in

the presence of ozone. From the high molecular-weight peak, with a peak molecular

mass (log Mp) = 5.3, a part of the pulp cellulose fraction was degraded and the

maximum shifted to the second cellulose peak, having a log Mp = 4.7. In the case

of PZ treatment, the shape of the MWD was virtually unaffected by the ozone

charge (Fig. 7.115).

It is well known that ozone treatment of pulp introduces carbonyl groups into the

AHG unit along the polysaccharide chain (see Tab. 7.41an d Section 11.3.2.2.2). In

a subsequent alkaline hydrogen peroxide stage (P), depolymerization of the oxidized

polysaccharide components in the pulp (cellulose and hemicellulose)

is favored due to b-elimination reaction. The high alkali instability of Z-treated

844 7Pulp Bleaching

3 4 5 6 7

6 kg O

/odt

4 kg O

/odt

2 kg O

/odt

ZP-sequence

dW/d(log M)

log Molecular Weight

3 4 5 6 7

6 kg O

/odt

4 kg O

/odt

2 kg O

/odt

PZ-sequence

dW/d(log M)

log Molecular Weight

Fig. 7.115 Differential MWDs of beech sulfite dissolving

pulps prepared by TCF bleaching applying different amounts

of ozone with Z-stage before (upper) (ZP) and after P-stage

(lower) (PZ), applying identical conditions in each stage

[123].

7.5 Ozone Delignification 845

pulps is also the reason why pulp viscosities of PZ-treated pulps are quite comparable

to those of ZP-treated pulps (despite the significantly higher MW and MN

values determined by GPC measurements), provided that the pulps are not subjected

to sodium borohydride reduction prior to viscosity measurements. Although

OPZ bleaching results in superior strength properties, an OZP-sequence

is preferred because of a significantly better brightness stability upon heat or light

exposure. This better brightness stability is achieved by partly oxidizing the carbonyl

groups that are introduced during ozonation. Brightness stability can also

be improved by reducing the carbonyl groups with sodium borohydride after ozonation

[92].

The effect of placing Z-stage on the generation of functional groups as a function

of ozone dosage is discussed in detail in Section 11.3.2.2.2.

Interestingly, in an ECF-sequence comprising O-, Z-, and D-stages, OZD was

found to be more selective than ODZ [124]. This can be explained by the fact that

chlorine dioxide bleaching following a Z-stage shows no adverse effect on cellulose.

The brightness stability after OZD is lower than after OZP bleaching, since a

final chlorine dioxide treatment is less effective in oxidizing or removing carbonyl

group-containing material. Contrary to a treatment in two separate bleaching

stages, a sequential application of chlorine dioxide (D) and ozone (Z) without

intermediate washing was shown to be very selective in delignifying softwood

kraft pulp [125]. This means that the Z- and D-stages are combined into one treatment

(DZ). (DZ) has been found to be more effective for unbleached pulps,

whereas (ZD) seems to be superior for oxygen-delignified kraft pulps [126]. In the

latter sequence, chlorine dioxide partially stabilizes the carbohydrate chain against

alkaline peeling reactions due to oxidation of the carbonyl groups introduced by

ozonation. In the case of an unbleached hardwood kraft pulp, however, chlorine

dioxide reacts with free phenolic groups before the highly reactive ozone is introduced,

the conclusion being that reaction kinetics clearly favors the (DZ) approach

relative to the (ZD) treatment [127]. Furthermore, after chlorine dioxide treatment

the pulp suspension is sufficiently acidic for a subsequent ozone stage. The (DZ)

concept is also advantageous with respect to AOX formation, as ozone has the

ability to destroy some AOX generated during D bleaching. Chlorine dioxide may

act as a radical scavenger, suppressing the extent of radical reactions during the

subsequent ozone treatment. However, in another study it was shown that the

selectivity was not impaired when washing was carried out between D and Z, thus

showing that the presence of residual chlorine dioxide seems not to be essential

for maintaining a high viscosity [128]. The actual reasons for improved selectivity

of a (DZ) treatment remain to be elucidated. In full ECF bleaching sequences, the

replacement of a D0 stage by (DZ) stages was shown to be particularly efficient,

since in the case of a hardwood kraft pulp 1kg charged (consumed) ozone could

replace 1.58 kg charged chlorine dioxide, as shown in Tab. 7.43 [128]. Ozone is

added to the pulp suspension 10 min after the introduction of chlorine dioxide.

846 7Pulp Bleaching

Table 7.43 Comparison of different ECF bleaching sequences of

a hardwood kraft pulp where DO is substituted either by Z or by

(DZ) stages according to [128].

Дата добавления: 2015-10-21