Concurrent Bleaching and Metal Management by Addition of EDTA to Chlorine Dioxide and Ozone Stages-Degradation of EDTA and Formation of...", 1996 International Pulp Bleaching Conference Proceedings

Closing the loop in ECF bleach plants requires a decrease in the consumption of chlorine dioxide. For easily bleached pulps with low kappa numbers the decrease can be achieved by switching from a O D(EOP)DED to a O QPDP or O QPZP sequence. For pulps with higher kappa numbers more delignifying power is required. With proper conditions the Q stage can be used as a (DQ) stage. In this way the same tower can be used both for delignification and metal management for the following peroxide stage. Similarly the Z stage can be run as a (ZQ) stage. In lab trials with (DQ)P bleaching using a softwood pulp optimum peroxide stability was achieved at a charge of 15 kg chlorine dioxide ptp (active Cl). Acceptable peroxide residuals were achieved with charges between 10 and 25 kg chlorine dioxide ptp in spite of an extensive degradation of EDTA at the highest chlorine dioxide charge. Lab trials with (ZQ)P bleaching using a hardwood pulp showed that the peroxide residual increased with an increase in ozone charge from 1 to 5 kg ptp in spite of an increased degradation of EDTA. The results show that improved bleaching sequences can be introduced in old bleach plants without the need for extra bleach towers or washing equipment.