Bleaching of eucalyptus kraft pulps with chlorine dioxide: Factors affecting the efficiency of the final D stage
2012 (English)In: TAPPI Journal, ISSN 0734-1415, Vol. 11, no 3, 43-53 p.Article in journal (Refereed) Published
We investigated the bleaching efficiency of the final chlorine dioxide (D 2) stage in the D 0(EP)D 1D 2 bleaching sequence, focusing on the effect of pH in relation to the bleaching history of pulp samples. The samples used were unbleached kraft Eucalyptus grandis pulps with kappa no. 14.8 and the same pulp oxygen-delignified to kappa nos. 12 and 9.8. The samples were bleached according to the D 0(EP)D 1 sequence to a brightness of about 86% ISO and then submitted to the final D 2 stage under identical conditions (e.g., chlorine dioxide charge, time, temperature, and final pH). The target final brightness was 90.5% ISO. Changes in the kappa number, brightness, viscosity, and contents of hexenuronic acid, 4-O-methylglucuronic acid, and the total amount of carboxylic acid (COOH) groups in pulps were monitored during the bleaching sequence. The final brightness of eucalyptus kraft pulps increased when the terminal pH of the D 2 stage was raised from 3.0 to 6.5. The 90.5% ISO target brightness in the D 2 stage was achieved for all pulps within a pH range of 4.5-6.5, but this required adjusting final pH for individual samples. The optimal pH value with respect to pulp viscosity was between 3 and 5. Despite dissimilar conditions applied in previous bleaching stages, the samples after the D 1 stage revealed similar residual lignin contents as shown by kappa number analysis. The content of hexenuronic acid in the samples, however, varied broadly, from 2 to 26 mmol/kg. Conductometric titration showed different amounts of carboxylic acid groups in pulps after the D 1 stage, of which hexenuronic acid accounted for only a minor part. The variations in the fiber charge resulted from the different bleaching conditions applied before the D 2 stage. The fiber charge affected the alkali demand in the final D 2 stage, whereas variations in the alkali demand affected the initial pH and associated process kinetics. Lower total fiber charge was found to be beneficial for improved final brightening and viscosity when bleached at higher final pH. Application: This study will help pulp mill engineers to better understand how a pulp's oxidative history may affect the process kinetics in the final D bleaching stage. Fiber charge is important when selecting optimal pH for the final brightening of eucalyptus kraft pulps.
Place, publisher, year, edition, pages
2012. Vol. 11, no 3, 43-53 p.
Bleaching condition, Bleaching efficiency, Bleaching sequence, Carboxylic acid groups, Chlorine dioxides, Conductometric titrations, Effect of pH, Eucalyptus grandis, Eucalyptus kraft pulp, Fiber charges, Final pH, Hexenuronic acid, Identical conditions, Initial pH, Kappa numbers, pH range, pH value, Process kinetics, Pulp samples, Pulp viscosity, Residual lignins, Unbleached kraft, Carboxylic acids, Cleaning, Fibers, Kraft process, Kraft pulp, Luminance, Optimization, Paper and pulp mills, pH effects, Unbleached pulp, Bleaching, Chlorine Dioxide, Kraft Pulps, Paper Mills, Ph, Samples, Unbleached Pulps
Chemical Sciences Chemical Engineering
IdentifiersURN: urn:nbn:se:kth:diva-99520ISI: 000313948200005ScopusID: 2-s2.0-84860264412OAI: oai:DiVA.org:kth-99520DiVA: diva2:542419
QC 201208012012-08-012012-07-312013-02-21Bibliographically approved