Paper Bleaching Chemicals

When chemical pulp, mechanical pulp and deinked recycled fibers are used in the production of white papers e. g. printing and writing papers, hygiene paper (tissue) or white top of board, these pulps have to be bleached up to a certain brightness. The bleaching of chemical pulp (see Volume 1) is carried out in several stages, with the elimination of residual lignin as far as possible being the primary objective of the first stage. The subsequent bleaching stages are responsible for brightening the pulp. Bleaching chemicals can be classified according to the particular groups they prefer to react with. Cations or radicals in acid medium react primarily with phenolic structures of lignin, alkaline nucleophiles attack carbonyl groups as fol¬lows:

. • Group I: Cl2, O3, peroxyacids react with all aromatic lignin units, with phenolic groups and their double bonds.
. • Group II: O2, ClO2 react primarily with free phenolic hydroxy groups.
. • Group III: H2O2 in alkaline conditions react mainly with certain functional groups in lignin, e. g. carbonyl groups.

Accordingly, bleaching is divided into pre-bleaching and final bleaching. After the basic chemical pulping process the following bleaching chemicals are added sepa¬rately in several steps, for which certain abbreviations have been introduced [4, 6].