Peracids in kraft pulp bleaching:: Past, present, and future

The main role of peracids in wood chemistry has long been in analytical applications. The TCF bleaching of chemical pulp has progressed rapidly during the 1990's. A lot of research has been carried out with different peracids. Industrial plants for peracetic acid production have been constructed and peracetic acid bleaching is currently in use, either permanently or occasionally, at several mills. The advantage of peracids over other oxygen-based bleaching chemicals is that they react selectively with lignin and leave pulp carbohydrates practically intact. The main parameters affecting the delignification kinetics are reaction temperature, peracid charge and pH. Peracids cause significant demethylation of pulp lignin leading to quinone and muconic acid type strictures and to lower a biphenyl and biphenyl ether structure content than oxygen delignification. These chemical modifications activate the pulp lignin and improve pulp bleachability. A peracid bleaching sequence yields better quality pulp than an ozone sequence because of the lower content of alkali-labile carbonyl groups. The future of peracetic acid, which is already a serious competitor to ozone as a delignification agent, depends greatly on its cost.