USING BLEACHING STAGE MODELS FOR BENCHMARKING SOFTWOOD ECF BLEACH PLANTS

Steady-state bleaching delignification and brightening models were used to gauge how well elemental chlorine-free
(ECF) bleach plants were using chlorine dioxide to bleach 25-kappa softwood brownstocks. Case 1 examined the
D0(EOP)D1 portion of Mill 1’s five-stage sequence that brightens the pulp to 86% ISO. Case 2 studied the D0(EO)D1
portion of Mill 2’s four-stage sequence, which brightens the pulp to 82% ISO, and Case 3 re-examined the same
bleach plant several years later after it made improvements around the extraction stage. The models highlighted days
in the above cases where high bleach usage occurred, presumably because of high brownstock and/or extraction
washer carryover, and days where bleach usage was normal. In Case 2, the model estimated that 10 kg of the 44 kg
chlorine dioxide/t pulp consumed in bleaching was likely reacting with washer carryover sources; approximately twothirds
of this extra consumption was assumed to be reacting with extraction filtrate. Changes that Mill 2 made (Case
3) reduced the unproductive chlorine dioxide usage from 10 to 5 kg/t pulp. When the delignification and brightening
models were simultaneously solved, the models predicted somewhat different optimized distributions of chlorine
dioxide to D0 and D1 versus actual values used in bleach plants. However, the forecasted chlorine dioxide totals agreed
with the actual values when washer carryover sources were considered. This study showed the bleaching models
could be used as hypothetical benchmarks for softwood ECF bleach plants.

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