Total Chlorine-Free Bleaching of Eucalypt Kraft Pulp, 1990 Pulping Conference Proceedings

Increasing pressures from the society are demanding drastic changes in the way chemical pulp is bleached. More than eliminating the use of chlorine gas, there is a clear trend to avoid the use of any chlorine compound in the bleaching of pulp. This paper discusses some preliminary results in the development of a chlorine-free bleaching sequence of eucalypt kraft pulp.

The study was developed around a sequence involving only oxygen and hydrogen peroxide, and a key intermediary acid treatment. The sequence can be described a s OA(E+P+O)P, where A stands for the acid treatment.

A number of different acids with potential use in the pulp mill have been tried in this process. Ozone was also evaluated, since the reaction is preferably performed in acidic medium. The bleaching sequences were performed aiming at 80%+ IS0 brightness, from a kappa=20 pulp, and compared with a reference chlorine containing bleaching sequence.

The results demonstrated the significant influence of acid treatments on metal removal, and upon improvements in pulp reactivity in the additional alkaline stages. The brightness targets were achieved with extremely high chemicals consumption, and significant effects on pulp quality. Further improvements are expected from optimization of the acid treatment and peroxide usage, combined with slight modifications in the pulping process.