Rationalization of the Use of TAED During Activated Peroxide Delignification and Bleaching. Part Ⅱ: Bleaching Optimization

摘要

Hydrogen peroxide (H_2O_2) delignification of kraft pulps activated by tetraacetylethylenediamine (TAED) was studied under various reaction conditions and optimized with the help of kinetic modelling of the pure TAED-H_2O_2 reaction system. In alkaline medium, the generation of peracetate is instantaneous, but it is decomposed rapidly by alkaline hydrolysis; TAED-H_2O_2 delignification was not superior to H_2O_2 alone at an elevated temperature (90℃), but better at a lower temperature. In acidic conditions, the rate of peracetic acid (PAA) generation is a limiting step. A good procedure waspre-generating PAA in a separate mixture under controlled conditions. Delignification at pH 6.5 and 90℃ was more efficient than a standard alkaline P stage, but excess H_2O_2 had a detrimental effect on cellulose. Sodium pyrophosphate protected cellulose but also penalized delignification. Finally, combining an acidic TAED-H_2O_2 treatment using sodium pyrophosphate, with a subsequent alkaline treatment without intermediate washing allowed an improved delignification while preserving pulp viscosity.