Mechanical Pulp Bleaching (Brightening) With Hydrogen Peroxide, 1998 Pulping Conference Proceedings

Before launching into the chemistry and application of mechanical pulp bleaching with hydrogen peroxide, some definitions may be in order. In today’s presentation I will be describing a process that is referred to as “bleaching” but is really “brightening”. The difference lies in the areas of overlap in the language used for “bleaching” chemical pulps and that used for mechanical pulps. In the treatment of chemical pulps, “bleaching” refers to the removal of lignin and subsequently increasing the brightness of the remaining cellulose. In mechanical pulps, the removal of lignin is not the objective, the brightening of the lignin-cellulose matrix is. This is just to give you an understanding of the difference, and why some chemical pulp bleachers may try to tell you that what you are doing is not really bleaching but brightening (which is correct). However, since nearly everyone in production of mechanical pulps refers to this process as bleaching and to the equipment used as a “bleach plant”, that is how it will be referred to here.

The main objective in mechanical pulp bleaching is to increase the brightness of the pulp while preserving the lignin-cellulose matrix, and most importantly, the pulp yield. To accomplish this the industry uses primarily two bleaching agents: sodium hydrosulfite (a reductive bleach) and hydrogen peroxide (an oxidative bleach).

This tutorial will focus on the oxidative bleaching agent, hydrogen peroxide, and how to use it to achieve brightness increases in mechanical pulps.