TCF Bleaching of Softwood Kraft Pulp. Metal Management Procedures
The aim of this work has been to increase knowledge of the influence of the metal ion content in softwood kraft pulp on peroxide bleaching and to investigate the effects of different metal management procedures at various stages in the production of TCF pulps (i.e., prior to cooking, prior to oxygen delignification and prior to hydrogen peroxide bleaching). In addition to chelation with EDTA (Q) and an acid pretreatment at pH 2 (A) an ion exchange pretreatment [(Mg+Ca)] under slightly acidic conditions was studied. In the ion exchange pretreatment, manganese ions are displaced from the wood material or pulp by an excess of magnesium and calcium ions. Chelation with EDTA, acid pretreatment at pH 2 and ion exchange pretreatment removed 94-98% of the manganese present in the original wood shavings and pulps. The results show that no major changes in the affinity of the manganese to the fibre material occur during delignification.