Correlation of COD and Yield in Chemical Pulp Bleaching, 1998 Breaking the Pulp Yield Barrier Symposium Proceedings

A correlation between the COD of the effluent and the yield loss can be made based on theoretical considerations using the lignin content and the oxidation equivalents applied. These data are compared to the results of experiments with different bleaching sequences. The effect of cellulose oxidation is similarly calculated and the result of an oxidative treatment is measured.

Normal ECF conditions were used to establish a best case scenario. For different oxygen delignified pulps a very similar ratio for the effluent COD and the yield loss was found. The ratio of the COD to the yield loss (both values in kg/t) was found to be 0.9. The application of drastic conditions in the extraction stages combined with high charges of oxygen or hydrogen peroxide produces a higher effluent load and a lower yield. Very high extraction temperature with higher peroxide addition degrades more cellulose. This shifts the COD to yield loss ratio more towards 1. The yield loss increases for oxygen delignified hardwood pulp from 20 kg/t (2%) to 25 kg/t and for softwood pulps from 40 kg/t (4%) to a level of 50 kg/t.