In a c stage was detected as aox in the effluent (50 kg Cl2 t–1 pulp generated

about 5 kg AOX) [10]. The different reaction of hypochlorite with lignin led to the

generation of only about half of the AOX for the same amount of active chlorine.

Chlorine dioxide reacts with lignin dominantly as an oxidant. Typically, only 0.5–

1% Of the active chlorine is converted into halogenated compounds (50 kg active

chlorine would generate about 250–500 g of AOX). There appeared to be no correlation

between the AOX value and effluent toxicity [11], and whilst some toxic

compounds contain halogen atoms, they are not all necessarily toxic.

During the 1980s, however, the detection of polychlorinated dioxins and furans

In chlorination effluent [12] led to the relatively rapid development of alternative

bleaching processes. Chlorine was eliminated from most bleaching sequences,

the initial intention being the complete replacement of all chlorinated compounds

(termed “totally chlorine-free”, or TCF bleaching). This could be easily achieved

with sulfite pulps, which typically have good bleachability. The conversion of sulfite

bleaching sequences from CEH (or CEHD) to shorter two-stage processes

with PP stages took only a few years, and put an end to discussions about the relevance

of AOX quantities in the description of potential or real hazards. The

absence of chlorinated products in the effluent allowed for effective biodegradation.

A sulfite mill having a lower brightness target applied the option to use magnesium

oxide as an alkalization source for the peroxide stage. This permitted

countercurrent water flow from the bleach plant to the pulping chemical recovery,

which in turn led to very low amounts of organic material remaining and the mill

being described as a (nearly) closed-cycle process [13].

Kraft pulp mills have been converted predominantly to elemental chlorine-free

(ECF) bleaching, with such processes using modified pulping and oxygen delignification

to achieve low residual lignin levels. In bleaching, chlorine dioxide and

oxidative supported extraction stages (Eop and Ep) are applied alternately, which

results in sequences such as DEopDEpD or DEopDP. Some mills use ozone to

further reduce the demand for active chlorine as chlorine dioxide, and this has

resulted in very low AOX discharge levels, whilst maintaining the pulp quality.

Depending on the intensity of the use of other chemicals and the temperature

applied in the chlorine dioxide treatment, ECF bleaching can become ECF-“light”

bleaching. This is reflected in the detectable residuals of halogenated compounds

1000 10 Environmental Aspects of Pulp Production

10.2 A Glimpse of the Historical Development

remaining in the fully bleached pulp. Such residuals are analyzed in similar manner

to AOX, and labeled as OX (“halogenated residual”). An ECF-“light” pulp can

have a residual of “OX” comparable to the “OX” background value of halogenated

organic material present in TCF pulp.

TCF bleaching of kraft pulp is much more complicated because the condensed

lignin is difficult to bleach without effective electrophilic oxidation. The compounds

available – ozone or peracid (peracetic acid or Caro’s acid) – are much less

selective than chlorine dioxide, and cannot be applied in large amounts without

risking fiber damage and pulp yield loss. Thus, high brightness or top strength

targets are typically difficult to meet simultaneously.