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.
Effects of washer efficiency on characteristics of effluents from D(EPO)D bleaching of a softwood kraft pulp
This paper describes the results of a systematic study of AOX, BOD, and COD released by bleaching southern pine kraft pulp in the Do(EPO)D_ sequence at a kappa factor of 0.20 and a D_ stage C102 charge of 1.2%. Measurements of all three parameters were made at three different levels of carryover of Do stage filtrate into the (EPO) stage and three different levels of carryover of 0EPO) stage effluent into the D_ stage, and the entire experiment was duplicated. The combined AOX contributions of the Doand (EPO) stages amounted to 11.6% of the chlorine atoms applied in the Do stage, while the contribution of the D_ stage amounted to 2.7% of the chlorine atoms applied in the Dh stage. The combined AOX contributions of all three stages decreased when the Do stage carryover was increased from 0 to 10% and was unaffected by 0EPO) stage carryover. With perfect washing between stages, COD contributions to the combined bleach plant filtrates by the Do, (EPO) and Dh stages were, respectively, 14.1, 39.8, and 2.9 kg/odumt. The combined COD contributions of all three stages was relatively unaffected by a low level (5%) of Do stage carryover, but a higher level (10%) resulted in an overall COD increase of 2 kg/odumt. Increasing (EPO) stage carryover from 0 to 30% decreased total COD by 3 kg/t. With perfect washing between stages, BOD contributions to the combined bleach plant filtrates by the Do, 0EPO), and D_ stages were, respectively, 3.8, 7.0, and 2.2 kg/odumt. No significant effect of carryover levels on the total BOD release could be detected. INTRODUCTION Kraft pulp bleaching sequences that do not make use of molecular chlorine are rapidly becoming the norm throughout the world, and are virtually mandated in the United States by the Environmental Protection Agency's "Cluster Rules.''_ One important sequence type relies on chlorine dioxide, C102, to accomplish both bulk delignification in the first stage and brightening in the latter stages. Detailed knowledge of the environmentally relevant characteristics of the effluents these sequences generate is needed to aUow them to be evaluated for specific applications. The characteristics of the filtrates from individual stages are of less interest than those of the entire bleach plant effluent that results when the filtrates from all stages are combined before being sent to the effluent treatment system. Because the byproducts of any bleaching stage are subjectto possible destruction in a subsequent stage, it is reasonable to expect that the characteristics of the combined effluent may be determined not only by the products of the reactions between the bleaching chemical and pulp components in each stage, but also bythe proportion of these products that are carried with the pulp into the following stage, as opposed to being discharged directly to the sewer. Consequently, it is of interest to determine the effect of bleach plant washer efficiency on effluent characteristics, and important to recognize it in laboratory evaluations of bleaching sequences from an effluent quality standpoint. In the present study, we determined properties of individual stage and combined filtrates from laboratory bleaching of a southern pine kraft pulp in the D(EPO)D sequence.
Changes in the lignin structure during a totally chlorine free bleaching sequence
En vue de mieux comprendre comment la lignine residuelle des fibres dans la pâte kraft est attaquee et degradee durant le procede de blanchiment, nous avons etudie une sequence de blanchiment utilisant le blanchiment a l'oxygene (O), un traitement avec chelateur, EDTA (Q), et un stade au peroxyde d'hydrogene alcalin (P). Nous avons ensuite isole et analyse, a l'aide de differents moyens, la lignine residuelle provenant de differentes pâtes et la lignine dissoute au plan des differents stades de la sequence de blanchiment. Nous avons observe que le blanchiment realise selon cette sequence donnait lieu a une elimination partielle de la lignine residuelle, accompagnee d'une elimination importante des groupes chromophores. Il semble que la structure chimique de la lignine qui reste dans les fibres apres la sequence OQP n'est que tres peu affectee par le traitement. L'oxydation s'est surtout traduite par une augmentation du nombre de groupes hydrophiliques, mais la lignine a conserve, dans une certaine mesure, son caractere phenolique et la structure aromatique etait elle-meme preservee. Les resultats que nous avons obtenus laissent croire que la lignine residuelle dans la pâte blanchie par la sequence OQP reste reactive vis-a-vis des agents de blanchiment oxydatifs, et qu'une optimisation accrue des conditions de blanchiment demeure possible.