New Predictive Models for COD from All Stages in Full ECF Bleaching Sequences
This paper proposes new mathematical models for the prediction of chemical oxygen demand (COD) from different stages in full ECF bleaching sequences and discusses several important issues related to model development and interpretation. The effects of different process conditions, like pH, chemical charge, initial Kappa number, carryover, temperature etc., on the formation of COD from the different chlorine dioxide stages (Do, D1, D2) and extraction stages (E, EO, EP, EOP) were studied and corresponding models were developed for softwood and hardwood pulps. The models, based on results obtained from the extensive studies of the kinetics, stoichiometry, and effluent properties of the chlorine dioxide stages and extraction stages in the ECF bleaching of pulp, were tested for accuracy on a vast range of data both from literature and laboratory experiments. The kinetics, stoichiometry and effluent properties have not previously been considered together for optimization and would be of considerable use in bleach plant optimization as well as in process control of different stages. This study will also help in understanding the need for modifications of the conventional conditions in both the chlorine dioxide and the extraction stages to obtain the best mix between a low formation of COD and high bleaching efficiency.
The Correlation of COD and Yield in Chemical Pulp Bleaching
1. Abstract 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.
A bleaching process for minimizing AOX discharges
A concept has been explored in which the organo-chlorine compounds generated in one stage of a bleaching sequence are destroyed in following sections of the process. Laboratory experimentation on eucalypt kraft pulp has demonstrated that full brightness may be achieved without deleterious effects on pulp strength and optical properties while reducing the discharge of these organo-chlorine compounds. The concept is accomplished in practice by not washing the pulp between the «chlorination» stage and the alkaline extraction stage. Extraction and chemical dechlorination of organo-chlorine compounds occur simultaneously