This project. In a Canadian project, the target was an effluent-free pulp mill, and

Water consumption and the bleach plant configuration and operation were modi-

A Glimpse of the Historical Development 999

fied stepwise over a number of years [8]. During these investigations, many lessons

Were learned about corrosion and bleaching efficiency in narrow loops, and

It became obvious that the bleaching process was extremely difficult to operate in

a “highly closed” mode.

The biological treatment of an effluent from chlorine bleaching, for example

With CEH or CEDED, is of limited effectiveness. Some of the halogenated compounds

produced are toxic, and most are poorly biodegradable [9]. Their amounts

Could be measured using absorption on to activated carbon. After washing to

Remove inorganic chloride, combustion of the loaded carbon provides an indication

Of the amount of absorbable organically bound halogen (AOX). The use of

Chlorine in the bleaching process led to the generation of very large amounts of

organically bound chlorine. As a rule of thumb, about 10% of the chlorine applied

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–

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

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