Optimization of Chlorine Dioxide Consumption in ECF Bleaching, 1999 Pulping Conference Proceedings

Olli Dahl, Tapio Tirri & Jouko Niinimäki

University of Oulu

Kari Lampela

Valmet Automation Kajaani Ldt.

Juha Fiskari

In this study the reagent consumption in D-stages of ECF bleaching was examined, keeping the dosages of alkali constant in the E1-stage and in the short alkali treatment (Es) stage between D1- and D2-stages. The ECF sequence examined in this study was D0-E1-D1-Es-D2. All tests were done in laboratory conditions using three different dosages of chlorine dioxide in each D-stage: in D0-stage the dosages were 15, 25 and 35 kg ClO2 as active chlorine per oven dry ton of metric pulp, in D1 and D2-stages 15, 20, 25 and 7.5, 10, 12.5, respectively. In all, 27 bleaching sequences were tested. In addition, three sequences were selected and re-tested with optimized pH in each chlorine dioxide stage.

The results indicate that full brightness of softwood kraft pulp (>88% ISO) can be achieved by using the major part of chlorine dioxide in the D0- or D1-stage and by optimizing the final pH of all D-stages. When D1-stage emphatic sequence (15-25-12.5) was used, a final brightness of 87.5% ISO and a total reagent consumption of 51.6 kg/tp (as kilogram equivalent chlorine per metric ton of oven-dry pulp) were attained. Correspondingly, with D0-stage emphatic sequence (25-20-10) the obtained brightness was 88.6% ISO and reagent consumption 54.0 kg/tp. The highest brightness 88.8% ISO and highest reagent consumption 54.7 kg/tp were attained with a strongly D0-stage emphatic sequence, 35-15-7.5. A strongly D0-stage emphatic sequence also gave the best final viscosity of the pulp.