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

Environmental Aspects of Pulp Production

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.

In 1990, only about 5% of the world’s bleached pulp was produced using ECF

bleaching sequences, but by 2002 this level had increased to more than 75%, representing

64 million tons of pulp [14]. The level of pulp still bleached with chlorine

Has been affected by a slower than anticipated conversion of pulp mills in

Japan, and its relative high use in the multitude of small non-wood pulp mills in

Asia (mainly China and India). The huge kraft pulp mills in Western Europe, in

The Americas, in Australia and New Zealand, as well as those in South Africa, are

operating in the ECF mode at a level above 90%. These mills reach annual pulp

Production rates of between 400 000 and 1 000 000 tons. The mills that still use

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