Improved Bleaching by Means of Fractional Washing, 2000 Pulping / Process & Product Quality Conference Proceedings

O. Pikka, J. Vehmaa, J. Alastalo, J. Gullischsen--This paper studies the bleaching chemical consumption at various impurity levels. The aim of the study was to find out how fractional washing can improve bleaching.

Fractional washing gives an opportunity to use two different washing liquids and to separate washing filtrates with different physical properties in a single washer. The filtrates can then be used separately: the more contaminated filtrate fraction can be removed from the process and the cleaner fraction of the filtrate can be used in other washing stages.

On the basis of computer simulation data a series of different D0-EP-D1 laboratory bleaching sequences with different contamination degrees were carried out. These bleachings showed a significant difference in the bleaching result when using different levels and different origins of COD in bleaching filtrates. In a mill trial the pre-bleaching wash press dilution filtrate was changed from D0 filtrate to hot water. As a result the final brightness of the pulp increased by 2 percentage points ISO, but the consumption of bleaching chemicals was reduced in each bleaching stage.

The study shows that fractional washing is the most effective way to decrease impurities in a closed bleach plant. By removing the most contaminated filtrate fraction, it is possible to reduce the bleaching chemical consumption without reducing the brightness.