Benefits of a Chelating Stage Prior to Peroxide Bleaching

Abstract

The pulping industry has been phasing out chlorine as a bleaching agent and nowadays only a minor part of the production of bleached paper pulp involves the use of chlorine gas. This development has been driven by environmental concerns, because of the formation and emission of chlorinated organic substances that follows the use of chlorine gas in pulp bleaching. The chlorine gas has, to a large extent, been replaced by chlorine dioxide, which generates much lower amounts chlorinated organics than chlorine gas. There are, however, even more environmentally benign bleaching chemicals available, such as hydrogen peroxide. Hydrogen peroxide is nowadays becoming a commonly used bleaching agent in chemical pulp mills. The bleaching efficiency of hydrogen peroxide is, however, dependent on the design of the bleaching sequence in which the peroxide stage is included. In this paper, the importance of a proper chelating stage (treatment with EDTA) prior to peroxide bleaching of eucalyptus kraft pulp is high-lighted. The results show that a high brightness at a high viscosity can be achieved at a relatively low consumption of peroxide, if the bleaching is proceded by a chelation stage. These results are in agreement with results found in the literature. Some results regarding the impact of the alkali addition in the peroxide stage are also included.