Introduction. Environmental restrictions for bleach plant effluents and the necessity to reduce

Environmental restrictions for bleach plant effluents and the necessity to reduce

the amount of organochlorine compounds (OX) in the pulp have driven the pulp

628 7Pulp Bleaching

industry to develop new environmentally benign delignification and bleaching

technologies. In this context, oxygen delignification has emerged as an important

delignification technology. The benefits of introducing an oxygen delignification

stage are manifold, and include a lower demand for bleaching chemicals in the

subsequent stages, a higher yield as compared to the final part of the cooking

stage, and the possibility of recycling the liquid effluents from an oxygen delignification

stage to the chemical recovery system to reduce the environmental impact

with respect to color, COD, BOD and toxic compounds (e.g., organochlorine) of

the bleach plant effluents. However, one of the major drawbacks of oxygen

delignification is its lack of selectivity for delignification in particular beyond 50%,

as this results in excessive cellulose damage which appears as a decrease in viscosity

and a loss of pulp strength. The significantly lower selectivity of oxygen-alkali

bleaching compared to conventional chlorine-based prebleaching sequences was

one of the reasons why the introduction of oxygen delignification to industrial

bleaching technology was not widely accepted by the industry. Figure 7.22 illustrates

the superior selectivity performance in terms of a viscosity–kappa number

relationship of a treatment with molecular chlorine followed by alkaline extraction

(CE) of a softwood kraft pulp as compared to oxygen delignification with and without

the addition of magnesium carbonate [1].

The potential of lignin degradation into water-soluble fragments by treatment

with oxygen in alkaline solution first became apparent during the 1950s [2–6].

Oxygen delignification was successfully applied to delignify and bleach birch and

spruce sulfite dissolving pulp. Several processes were patented during this early

0 10 20 30 40

unbleached pine kraft pulp

C-E pre-bleaching

oxygen bleaching at 100.C

Intrinsic viscosity [ml/g]

Kappa number

Fig. 7.22 Selectivity of oxygen delignification at 100 °C in

comparison to chlorination followed by alkaline extraction

(according to Hartler et al. [1]). The broken line denotes oxygen

delignification with no MgCO3 added.

7.3 Oxygen Delignification 629

phase of research, but were not commercialized because of the observed extensive

depolymerization of carbohydrates [7–9]. The poor selectivity has been explained

by the formation of reactive oxygen-based radicals (e.g., hydroxy radicals) generated

by oxygen attack on lignin structures [10]. A major breakthrough in oxygen

delignification occurred during the 1960s, when Robert and colleagues discovered

that the addition of small amounts of magnesium carbonate resulted in preservation

of the strength properties of paper-grade kraft pulp [11–13]. This opened the

door to the commercial development of oxygen as a delignifying agent. The first

installation was a high consistency oxygen delignification plant built in South

Africa at SAPPI’s Enstra mill in 1970. This investment was based on a successful

pilot plant operation in 1968 in Sweden, and was constructed as a cooperative

effort among SAPPI, Kamyr Inc., and Air Liquide. Reported high investment

costs and safety problems with the handling of combustible gases certainly

retarded the acceptance and implementation of this new technology.

The development of medium consistency, high-shear mixers during the early

1980s led to a rapid increase in the installation of oxygen delignification plants

due to its beneficial effects on the environment, process economy and energy savings

[14,15]. This process is also more amenable to retrofit in existing mills than

high consistency processes, and can be easily incorporated as intermediary stage

in the sequence, for example as combined with an E-stage [16]. In 1996, there

were more than 185 oxygen-delignification installations throughout the world,

with a combined daily production of about 160 000 t oxygen-delignified kraft pulp

[17,18]. The data in Fig. 7.23 show that 80% of these installations have come onstream

during the past 10 years, mainly driven by the stricter emission limits prescribed

by regulatory authorities.

As mentioned previously, oxygen delignification also provides an economic

attractive alternative to chlorine-based bleaching stages. It is reported that roughly

5 kg of oxygen can replace about 3 kg of chlorine dioxide. At a price differential of

0.61SEK kg–1 versus 8.6 SEK kg–1, respectively, the cost of using oxygen is about

one-eighth that of using chlorine dioxide [19]. Moreover, the energy requirement

to separate oxygen from the atmosphere is significantly less as compared to the

generation of chlorine dioxide, and this is a favorable prospect for the future [20].

Presently, oxygen delignification has become a well-established technology.

Because of selectivity advantages and lower investment costs, the medium consistency

technology (MC, 10–18%) has dominated mill installations for the past 10

years, although high consistency installations (HC, 25–40%) are also in use.

Recently, the industry has adopted the installation of two-stage oxygen delignification

systems to increase both the selectivity and efficiency of the treatment. A typical

50% delignification level has thereby been increased to about 65% for a softwood

kraft pulp with an unbleached kappa number between 25 and 30.

A detailed study of representative oxygen delignification installations worldwide

clearly indicates the advantage of a two-stage oxygen delignification system over a

one-stage system. The good performance of the high-consistency systems mainly

results from better washing before the oxygen stage.

630 7Pulp Bleaching

1970 1975 1980 1985 1990 1995

Capacity*103 [adt/d]

Year

one-stage two-stage

Fig. 7.23 Daily production capacity of oxygen-delignified pulp

on a worldwide basis [18].

For softwoods, evaluation of the database provides an average of 47.5% delignification,

ranging from 28% to 67%. The incoming kappa number to the oxygen

stage ranges from 32 to 22, and the outgoing kappa number from 22 to 8.5. For

hardwoods, the performance of oxygen delignification varied from 19% to 55%,

with an average of 40%. Kappa number variation is significantly reduced across

the oxygen stage, from a range of 12–22 at the inlet to 7.5–13.5 at the discharge.

The reason for these differences in unbleached kappa numbers of hardwood kraft

pulps depends on the greater variability of hardwoods with respect to optimum

yield and final pulp properties. For example, birch is often cooked to 18–20 kappa

number, while many eucalypt species are only cooked to 12–14 kappa number

[18]. In accordance with recent developments and the results from detailed investigations,

there appears to be a lower limit of kappa number in the bleach plant

for softwood kraft pulps of 8–10 and for hardwoods of 6–8 [19]. With continuing

progress in oxygen delignification technology, it is expected that in future the

cooking kappa will be raised to levels higher than 30, again because considerable

wood yield can be preserved [21]. The yield loss during the residual cooking phase

is significantly higher than during oxygen delignification. With the new highly

efficient multi-stage medium consistency technology available, the overall yield

can be increased by about one percentage point by increasing the cooking kappa,

for example from 20 to 25.

Delignification in the oxygen stage means a smaller decrease in yield than

delignification in cooking, as long as the degree of delignification in the oxygen

stage remains moderate. As a rule of thumb, the yield decrease in the oxygen

stage equals 0.1–0.2% on wood per 2 units of kappa number decrease, while in

7.3 Oxygen Delignification 631

cooking the yield decrease corresponds to 0.3% on wood for the same kappa number

reduction [22]. Thus, oxygen delignification is more selective in terms of yield

preservation than kraft cooking at kappa numbers corresponding to the final

phase of a low kappa kraft cook [23].

It is generally acknowledged that an O- or OO-stage can remove 35–50% of the

residual lignin in hardwood kraft pulp and 40–65% in softwood kraft pulp, without

significantly impairing the selectivity of delignification and the physical pulp

properties. The results of extended oxygen delignification studies indicate that distinct

yield benefits can be accomplished by interrupting the cook at a high kappa

number (e.g., 40–50) in the case of softwood kraft pulps include reference. The

subsequent oxygen delignification of the high-kappa number pulps has been

shown to provide 3–4% yield benefits over conventional cooking and bleaching

technologies. These observed yield benefits are then further amplified by reducing

the organic load on the recovery furnace

7.3.2

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