Technical Problems and their Measures for ECF Bleaching

Abstract

In Hachinohe mill of Mitsubishi Paper Mills Limited, the bleached kraft pulp (BKP) line named “3 BKP” was switched to an elementary chlorine free (ECF) bleaching in October 2000, and “2 BKP” was switched to the ECF bleaching in August 2005.
It was found that there were some differences as to bleaching behaviors between the ECF bleaching whose first stage was chlorine dioxide and a conventional chlorine bleaching. For example, the ECF bleaching was more susceptible to a brightness of oxygen-delignified pulp than the conventional one. Emissions of adsorbable organic halogen and of chloroform were decreased by introducing the ECF bleaching. Thus, environmental targets were achieved. However, technical problems as to the ECF bleaching operation and a pulp quality newly occurred. These problems were an increased bleaching cost, calcium oxalate scales on diffuser screen and a discoloration of pulps and so on. Measures to hexenuronic acid which is deeply related to the problems have to be pursued for a fundamental solution to the problems.
This paper described in regard to characteristics of the ECF bleaching, the technical problems and their measures that became apparent through the ECF bleaching operation.