ISSN ………… - International Network for Bamboo and Rattan

dioxide in later bleaching stages, it is possible to achieve high degrees of purification and brightness
without the degradation of cellulose.
The main disadvantage of the process is the extensive degradation of cellulose fibres as well as
introducing high pollutants in the effluent. Further, higher brightness beyond 80 per cent is practically
impossible while using hypochlorite. Alternatively, the use of chlorine dioxide can considerably
reduce the use of free chlorine and the chlorination stage which has the advantages of lowering the
consumption of oxidising chemicals as also sodium hydroxide at the extraction stages, improved pulp
properties such as more stable brightness, improved effluent properties such as lower colour, toxicity,
acidity, salinity, organic chlorides, COD and mutagenicity. The foremost advantage of using chlorine
dioxide bleaching is the benefit of attaining brightness levels in excess of 85 per cent with the
associated less degradation of cellulosic fibres. Practically all over the world chlorine dioxide
bleaching has been in use for the last four decades.
Multi-stage bleaching involves a series of stages, each complete within itself and operating under
conditions previously described. The procedure may vary from two stages to as many as ten,
involving chlorination, alkali extraction, hypochlorite, chlorine dioxide, and peroxide stages. The
symbols used to describe the sequences and the various sequences, in practice for the bleaching of
pulps are given in Appendix 6C, D &E. Pulps of different brightness may be obtained by varying the
amount of chemical added, sequence of stages, and number of stages. The sequence of operation of
the various stages will depend on the type of unbleached pulp, the quality of the bleached pulp, the
brightness desired, and the economics of the process.
In most commercial processes chlorination is still the first bleaching step, and is also called the prebleaching
stage. Chlorine converts the residual lignin in water- and alkali-soluble degradation
products, and is therefore generally followed by an alkaline extraction step to remove those
components.
4.3.1.2. Substitution for chlorine
As chlorination causes the largest number of environmental problems with the bleach plant effluents,
many attempts have been made to replace chlorine or to reduce the amounts of chlorine used or the
chlorinated products in the effluents. This can be accomplished by cooking the pulps down to low
kappa numbers, thus reducing the chlorinated organic load in the effluents, or by replacing part of
chlorinated lignin fragments. Replacing chlorine altogether by peroxide or oxygen bleaching enables
processing of effluents with few problems with regard to environmental protection.
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