Role of Metal Ions in TCF-Bleaching of Softwood Kraft Pulps, 1994 Pulping Conference Proceedings

M. Ek, P. Axegard, E. Bergnor, U. Ekholm

Metal ions present in wood and pulp have a great effect on the efficiency of bleaching softwood kraft pulps with oxygen and hydrogen peroxide. Great improvements in the bleaching efficiency of these chemicals have been obtained by adjusting the metal ion content in the fiber wall. Some metal ions e.g. magnesium have the ability to protect the cellulose against degradation.

Ozone and chlorine dioxide are less sensitive to metals than oxygen and hydrogen peroxide. Ozone and chlorine dioxide are comparable with each other as far as delignification and brightness efficiency go.

We have also found that ozone bleaching prior to hydrogen peroxide bleaching greatly improves the performance of the hydrogen peroxide stage. With an appropriate metal ion profile, laboratory bleached (AZQP) softwood kraft pulps can reach brightness levels above 90% ISO, with 5-10 % lower pulp strength properties and bleaching costs comparable to ECF-bleaching.