ASAM Pulping of Douglas-Fir and Chlorine Free Bleaching, 1991 Pulping Conference Proceedings

North American industrial Douglas-fin chips were pulped using both, the ASAM and the kraft process. Bleaching of the pulps was conducted in a conventional five stage C/D E D E D sequence. Additionally, the ASAM pulp was bleached in a completely chlorine free 0 Z PN short sequence.

Delignification in kraft pulping stopped at kappa number 30, whereas in AS AM pulping it was extended to kappa number 22. Nevertheless, the yield was superior to that of the kraft cooking. In conventional bleaching of the ASAM pulp yield losses were lower and roughly half of the chlorine demand of the kraft pulp was required. Consequently, the yield advantage of the ASAM pulp was further enhanced. The bleached pulps had the same tear strength but the tensile strength of the ASAM pulp was about 20 % higher.

Chlorine free bleaching of ASAM Douglas Fir pulp is possible in a 3 stage 0 Z PN sequence yielding high brightness and only marginal strength losses.