Bleaching of eucalyptus kraft pulps with chlorine dioxide : Factors affecting the efficiency of the final D stage

• 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.

Ämnesord

NATURVETENSKAP  -- Kemi (hsv//swe)

NATURAL SCIENCES  -- Chemical Sciences (hsv//eng)

TEKNIK OCH TEKNOLOGIER  -- Kemiteknik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Chemical Engineering (hsv//eng)

Nyckelord

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

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