Elemental chlorine-free bleaching

Figure 16.2a summarizes the major uses of chlorine. Chlorinated organic compounds, including 1,2-dichloro-ethene and vinyl chloride for the polymer industry, are hugely important. Dichlorine was widely used as a bleach in the paper and pulp industry, but environmental legislations have resulted in changes (Figure 16.2b). Chlorine dioxide, CIO2 (an elemental chlorine-free bleaching agent), is prepared from NaC103 and is favoured over CI2 because it does not produce toxic effluents. ...

Chemical pulp bleaching has undergone significant process changes since around 1990. Until that time, nearly every chemical pulp mill that had used bleaching had incorporated elemental chlorine (Clj) into some of its processes. Because of environmental and health concerns about dioxins, U.S. pulp mills now use elemental chlorine free (ECF) and total chlorine free (TCF) bleaching technologies. The most common types of ECF and TCF are shown in Table 21.5. The difference... 

Common Chemicals Used in Elemental Chlorine Free (ECF) and Total Chlorine Free (TCF) Bleaching Processes... 

Elemental chlorine free (ECF) bleaching with low AOX or totally chlorine free (TCF) bleaching... 

ECF [Elemental chlorine free] A generic term for pulp-bleaching processes which use chlorine dioxide and other oxidants in place of elemental chlorine. See also TCF. 

Nelson, P., Elemental Chlorine Free (ECF) and Totally Chlorine Free (TCF) Bleaching of Pulps, in Environmentally Friendly Technologies for the Pulp and Paper Industry, R. A. Young and M. Akhtar (Eds.), John Wiley Sons, New York, 1998. 

The case study presented here deals with a pulp and paper production process. The plant produces Bleached Eucalyptus Kraft Pulp, using the ECF process (Elemental Chlorine Free). The Kraft pulping process is performed in two different phases, which influence the final pulp quality the cooking process of wood chips (eucalyptus globules) followed by the pulp bleaching. The cooking process is the phase that most contributes to the preservation of the main pulp characteristics, which, in turn, will ensure high quality paper. 

Since it became clear that the direct use of chlorine in the bleaching of pulp and paper leads to highly poisonous chlorinated organic species in the waste water, chlorine dioxide is used instead in the so-called ECF (Elemental Chlorine Free) process. The competing process is the TCF (Total Chlorine Free) process, which uses peroxides [11],... 

Molecular chlorine, CI2, is an ideal chemical for delignification as it is cost-efficient and has reasonable selectivity, well capable of removing 75-90% of the residual lignin in a single stage. However its undesirable effects - waste water with chlorides that is corrosive, and a tiny by-product of chlorinated organic compounds -means that molecular chlorine has been abandoned entirely in some countries and where it is still used this is often in conjunction with chlorine dioxide (D) whose environmental footprint is some 2.6 times smaller. Today, more than 75% of pulps are bleached without any molecular chlorine. These are described as elemental chlorine-free (ECF). Only c. 6% of bleached pulps are total chlorine-free (TCF),... 

In paper industry jargon, chlorine dioxide bleaching is ECF bleaching (elemental-chlorine-free) and bleaching with oxygen species TCF bleaching (totally-chlorine-free). 

Fig. 16.2 (a) Industrial uses of CI2 in Western Europe in 1994 [data Chemistry Industry (1995) p. 832]. (b) The trends in uses of bleaching agents in the pulp industry between 1990 and 2001 CIO2 has replaced CI2. Both elemental chlorine-free and totally chlorine-free agents comply with environmental legislations [data Alliance for Environmental Technology, 2001 International Survey]. 

In all cases in modern pulp bleaching, elemental chlorine is excluded and only chlorine dioxide is applied to yield elemental chlorine-free pulps (ECF pulps). If any chlorine-containing compound is avoided totally chlorine-free pulps (TCF pulps) are achieved. Concerning the bleaching, Kraft pulp mills have been converted predominantly to elemental chlorine-free (ECF) bleaching. Totally chlorine-free (TCF) bleaching is the method of choice for sulphite mills. Therefore, the extent of TCF bleaching remains at around 5% of the pulp production. 

Figure 7.2. Effects of different bleaching sequences on the fibre surface content of lignin TCF, total chlorine free ECF, elemental chlorine free. The bleaching steps are oxygen delignification (O), ozone (Z), peroxide (P), chlorine dioxide (D) and alkali extraction (E). (From ref. (4) with permission)...

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