Mill assessment of mixing quality using laboratory bleaching studies

Good mixing is essential for economic production of high quality bleached pulp. However, it is difficult to evaluate mixing quality in the mill, and therefore, to estimate the impact a change in mixing could have on pulp quality and bleaching economics. Here, a method of measuring mixing quality based on laboratory bleaching studies and mill operating data is evaluated. The mill used in the study operates parallel Dn stages in its bleach plant. One Dn mixer was an older peg mixer, the other a 1980's vintage high-shear device. The absolute mixing quality for each stage was determined using the procedure detailed here. This allowed estimation of the chemical savings possible if mixing was improved. These values were compared with chemical use following the replacement of both mixers by newer high-intensity mixers. The technique described has the advantage of using the pulp itself as the mixing tracer. Unfortunately, the accuracy with which the bleaching response (kappa number) could be determined combined with the limited kappa drop across the Dn stage make it unsuitable for accurate mixing assessment here. Better success would be found in the Do stage.