ECF Bleaching of Oxygen-Delignified Softwood Pulp with the Minimum Charge of CIO2, 1993 Pulping Conference Proceedings

Kraft pulp mills in Finland and Sweden have changed over to ECF bleaching almost totally within the last few years. The move to ECF has been easy at mills with sufficient Cl02 capacity, but much more difficult elsewhere. In practice, the Cl02 charge varies greatly at Scandinavian mills (10 - 30 kg Cl02 /ADt), but is nevertheless a substantial cost factor.

KCL has carried out an extensive study on the optimization of ECF bleaching with the sequence D - EO(P)-D-E(P)-D to a brightness of 90% ISO. The kappa factor in Do was varied from 0.08 to 0.24, peroxide was tested in the E stages (0 to 4 kg/ADt) and the influence of xylanase treatment was included. The main objective was to find ways to minimize Cl02 consumption in ECF bleaching.

The results can be summarized as follows:

- The optimum kappa factor was surprisingly low (0.12) when an intensified EOP stage was included.

- When the EO stage was less efficient, the optimum kappa factor was higher.

- Cl02 consumption rose sharply below a kappa factor of 0.12 , and more slowly above the optimum.

- An enzyme treatment cut the Cl02 consumption by 15-20%.

- Under all conditions, ECF bleaching was at least l0-15% more expensive than D50 bleaching

- Pulp quality was basically unchanged.

ECF bleaching is relatively easy to apply. The most important control variables are kappa number of pulp, kappa factor, E1 stage conditions and chemical charges and enzyme usage.