MINIMIZING THE EFFECT OF UNDESIRABLE REACTIONS IN ECF BLEACHING

摘要

One of the undesirable reactions during ECF bleaching is the formation of resistant quinones resulting from the oxidation of lignin phenolic rings by C1O2. Occurrence of this side reaction was demonstrated by the implementation of a new in situ analytical method which measured a quinone number. This method is based on the selective reaction of free-phenol groups in pulp by C1O2 at low temperature (0°C) and on the reduction of quinones into free phenols by sodium dithionite. Although it cannot be claimed that the result is absolutely quantitative, it gives at least a valuable information on the quinone content. Its main advantage is that no lignin extraction is required. As during bleaching the subsequent chlorine dioxide stages cannot destroy these new quinone groups in an efficient way, it is proposed to apply a hydrogen peroxide treatment after each C1O2 (D) stage. Another undesirable reaction is the consumption of CIO2 by the muconic acid type structures which are the primary oxidation products of lignin during a D stage. Experiments with cis, cis-2,5dimethyl muconic acid, a model for lignin oxidized units, indicated that this reaction is most probable. Although phenolic moieties in lignin are more reactive than the muconic acid structures, the use of high C1O2 charges respective to lignin may favour its consumption by the muconic acid structures. One solution to this problem is to apply lower charges of C1O2 in each D treatment. The relevance of these findings is demonstrated by the high efficiency of (DEP)(DEP)... sequences where DEP stands for a C1O2 stage at low charge of chlorine dioxide followed by an alkaline hydrogen peroxide treatment.