ELEMENTAL CHLORINE-FREE BLEACHING OF SODA RAPESEED PULP

Soda rapeseed pulps delignified to kappa numbers of 21.4 and 16.4 underwent elemental chlorine-free bleaching using chlorine dioxide, hydrogen peroxide, and peracetic acid as bleaching agents under laboratory conditions. Three four-stage bleaching sequences, DE(P)PPaa, D0EPD1P and D0EPD1D2, were applied to investigate the properties of bleached soda pulps, namely their brightness, zero-span breaking length and degree of polymerisation. For comparison, an oxygen-predelignified kraft softwood pulp with an initial kappa number of 10.5 was bleached under the same conditions. Soda pulps subjected to bleaching processes comprising two or three D stages were found to have a final brightness of 83.1 to 83.8% ISO, which was lower than that of 87.6 to 87.9% ISO obtained for kraft softwood pulp. The decrease in fibre strength was of only 7-9% for soda pulps, slightly greater than that of kraft softwood pulp. In contrast to fibre strength, the loss in the degree of polymerisation was considerable, roughly of 35% for soda pulps and 40 to 47% for kraft softwood pulp, depending on the bleaching sequence.