Modified Polysulphide (AQ)-Pulping of Softwood, 1997 Pulping Conference Proceedings

A review of the polysulphide (PS) liquor generation methods has been made. By preparing some PS liquors which simulate future possibilities, pilot scale PS-(AQ) pulping studies were conducted. The studies were carried out in a recently acquired pilot plant which can simulate both conventional cooking techniques as well as the latest pulping developments. Two types of pulps were made, type 1 were liner board grade pulps and type 2 were bleachable grade pulps. Type 1 pulps were made by using up to 20% green PS-liquor in conventional PS-AQ pulping. Type 2 pulps were made by adding all PS in the impregnation stage of iso-thermal cooking simulated cooks and applying either NaOH (Soda liquor) or kraft liquor (WL) as the major alkali source.

The pulping experiments to produce liner board grade pulps showed that, addition of PS-green liquor could save up to 9 % polysulphide cooking liquor (orange liquor). The use of PS-green liquor had a negative influence on the yield increase attained by PS-AQ pulping applying 1% PS and 0.05% AQ. The yield gain was reduced from 4.4% to 3.5% (on od. wood).The paper making properties of the PS-AQ liner board pulps were not significantly influenced by PS-green liquor. All PS-AQ pulps had paper properties very similar to liner board grade kraft pulps. The yield increase by conventional PS-AQ pulping using 1% PS and 0.05% AQ was reduced by 50% when the pulping was extended from kappa number 80 to kappa number 30.

The pulping experiments of making bleachable softwood pulps of kappa number 30 showed that, addition of 2.2% PS (on od. wood) in the impregnation stage of simulated iso-thermal cooking cooks when using soda liquor as the major alkali source, needed an AQ charge of 0.05% to show a yield gain of 3% (on od. wood). Splitting the AQ addition between impregnation stage and cooking stage gave no significant change in yield increase or rate of delignification. By using a kraft liquor instead of Soda liquor as the main alkali source, there was a significant increase in the rate of delignification, but it gave only a minor improvement in pulp yield. The paper making properties of the bleachable grade pulps of kappa number 30 level were all very similar, except for the tearing strength which was somewhat reduced by PS-AQ pulping compared to iso-thermal cooking kraft pulping.