TCF Bleaching of ASAM Softwood Pulp, 1994 Pulping Conference Proceedings

O. Kordsachia, E. Oltmann, J. Odermatt, R. Patt

The objective of this investigation was to find optimal transition points between cooking and bleaching and between the different bleaching stages of an OZ(E)P sequence for ASAM pine pulp. Optimization within each bleaching stage and of the whole sequence shows that a kappa number of about 25 can be regarded as the best starting point with respect to bleaching response, yield and pulp strength. Oxygen delignification has to be conducted to a kappa number of about 8. In the ozone stage up to 1 % ozone is required to attain a kappa number between 2 and 3, which allows final bleaching to high brightness. The transition point between O and Z stage has not effect on the selectivity of delignification. Some ozone can be saved by using more peroxide in the final stage, but saving of 0.1 % ozone requires an additional charge of 0.5 % peroxide. At appropriate conditions peroxide bleaching is more selective and results in better viscosity and strength of the fully bleached pulp.

Further improvements of TCF bleaching of ASAM softwood pulps are discussed with special emphasis on ozone free bleaching with the OA(OP)P sequence.