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

• The bleaching efficiency of the final D stage in the D 0(EP) D 1D 2 bleaching sequence was investigated focusing on the effect of pH in relation to the bleaching history of pulp samples. The pulps used were unbleached kraft Eucalyptus grandis pulps with a kappa number of 14.8, and the same pulp oxygen-delignified to kappa numbers 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. The target final brightness was 90.5% ISO. Changes in the kappa number, brightness, viscosity and contents of hexenuronic acid (HexA), 4-O-methylglucuronic acid and the total amount of carboxylic acid groups in pulps were monitored during the bleaching sequence. The target brightness in the D 2 stage was achieved for all pulps, but this required a different final pH for individual samples, indicating dissimilarities between chromophore types in them. In general, brightness increased as the pH increased from 3 to 6. The optimal value with respect to pulp viscosity was between pH 3 and 5. Despite unlike 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 was, however, found to vary broadly in the range from 2 to 26 mmol/kg. Conductometric titration showed different amounts of carboxylic (COOH) groups in pulps after the D 1 stage, of which HexA accounted for only a minor part. Since the content of 4-O-methylglucuronic acid groups was similar for all samples, the difference in the amount of COOH groups was mainly associated with oxidation of the residual lignin. Some of the charges might also originate from oxidized cellulose. It was concluded that the variations in the fiber charge result from the different bleaching conditions applied prior to the D 1 stage.

Ämnesord

NATURVETENSKAP  -- Kemi (hsv//swe)

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

Nyckelord

4-O-methylglucuronic acid

Bleaching history

Brightness

ECF bleaching

Eucalyptus kraft pulp

Fiber charge

Final D 2 stage

Hexenuronic acid

Kappa number

pH

Viscosity

Kappa numbers

Bleaching

Carboxylic acids

Chromophores

Kraft process

Kraft pulp

Lignin

Luminance

pH effects

Unbleached pulp

Cleaning

Eucalyptus

Kraft Pulps

Lignins

Unbleached Pulps

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