Particle Removal with Chlorine-Free Bleaching Chemicals, 1996 International Pulp Bleaching Conference Proceedings

Elisabeth Bergnor & Pia Mellander

STFI, Swedish Pulp and Paper Research Institute

Maria Strom

KTH, The Royal Institute of

Technology,

This paper considers the efficiency of particle removal with chlorine-free bleaching chemicals when producing fully bleached softwood kraft pulps. Comparisons are made between bleaching with hydrogen peroxide (P), peracetic acid (T), ozone (Z), and chlorine dioxide (D). The reaction patterns for the bleaching agents were studied when shives and fibers were treated in separate experiments. Also, the efficiency of particle removal in TCF and ECF sequences were compared when shives or knots were added to particle-free pulp before bleaching. Our results show that hydrogen peroxide in a QP-sequence is very efficient in removing particles at a given delignification degree. Furthermore, in a microscopic study, we found that the reaction pattern for hydrogen peroxide in a QP-bleaching position is similar to that of chlorine dioxide, as the delignification seems to occur evenly across the shive. On the contrary, ozone bleaching of shives is an extreme surface reaction; hence, the particle thickness should be even more critical when bleaching with ozone than with other bleaching chemicals. In a QPQ(PO)* sequence, it is possible to obtain the same degree of shive elimination as with ECF bleaching. If an ozone or peracetic acid stage precede a final hydrogen peroxide stage, problems with pulp cleanliness may appear. However, mechanical treatment after final bleaching seems to improve shive elimination in all TCF sequences. This suggests that shive elimination in TCF bleaching can be as effective as in ECF bleaching.