Combining Chlorine Dioxide and Molybdate Catalyzed Hydrogen Peroxide for Improved Delignification of Canadian Hardwood and Softwood Pulp

摘要

Laboratory trials with Aspen and Spruce pulp samples were evaluated with the (DPmo) treatment at the start of an ECF bleaching sequence. This option consists of a short chlorine dioxide stage followed without intermediate washing by an extended acidic, molybdate catalyzed hydrogen peroxide stage. Several bleaching sequences were run and compared with a DEoDD sequence as reference. The aim was to assess the potential for reducing the chlorine dioxide demand and to determine the influences on brightness stability and fiber properties. A very impressive efficacy of the (DPmo) treatment was achieved for Aspen pulp. About 50 % of the normal chlorine dioxide charge could be replaced by only 5 kg H_2O_2/t + 500 ppm molybdenum. The final brightness target of 89 % ISO was easily achieved and the fully bleached pulp showed good brightness stability in accelerated heat aging. The mechanical fiber properties remained unchanged. For softwood pulp, the savings potential in chlorine dioxide consumption was significant, though lower than for hardwood. Furthermore, the bleaching conditions should be selected carefully to avoid harm to the pulp viscosity. The results demonstrate that a (DPmo) treatment is a useful tool to improve delignification, and not only for pulp mills facing a limited chlorine dioxide capacity.