Present and Future Bleaching of Secondary Fibers, 1994 Recycling Symposium Proceedings

Wolfgang Matzke, Harald Selder, Johannes Kappel

Brightness is one of the most important quality criteria for deinked pulp (DIP). A number of deinking plants for wood-containing and wood-free waste paper is equipped with a post-bleaching stage. For higher brightness requirements there exist several options:

a) usage of an upgraded waste paper furnish and/or

b) improval of the deinking process, e.g. by addition of post-flotation

c) improval of the bleaching process, e.g. by a) two-stage oxydative and reductive bleaching with peroxide and hydrosulphitc or FAS, i.e. totally chlorine-free b) raising the consistency.

In single- and two-stage bleaching a disperger can serve as a very efficient bleach mixer.

In some installations, after bleach mixing in a disperger at 30% consistency the bleaching reaction is continued in the medium consistency range at 12 - 15 %. An example will be given for the bleaching operation in the production of DIP with 72 - 74 % IS0 brightness from mixed office waste. High consistency mixing as well as bleaching are done in more recent installations, at least for the peroxide stage. New deinking installations, e.g. for LWC grades, can be operated at high consistency in both, the oxidative and the reductive stage. A reversion of the bleaching sequence, i.e. hydrosul-phite or FAS followed by peroxide, seems to be a favourable variant of the process.

Ozone (Z) can be helpful in bleaching coloured and brown fibers, e.g. in a furnish of mixed office waste. However, this would be a 3-stage process and only a minimum of mechani-cal pulp fibers is allowed. Peroxide intensified oxygen bleaching (EOP) could handle a higher content of mechani-cal pulp, e.g. from household collections. Again, a three-stage bleaching process would be needed. An optimized two-stage process with peroxide and hydrosulphite or FAS could be the better choice. Oxygen and ozone may, however, find a place in bleaching secondary fibers from old corrugated containers (OCC).