Optimization of ECF bleaching of kraft pulp: II. Effects of acid prehydrolysis on hardwood pulp bleachability

Earlier studies (1) have shown that when a hardwood kraft pulp is bleached in the D 0 (EO)D 1 ED 2 sequence, the brightness of the pulp emerging from the D 2 stage can be accurately predicted from the brightness of the pulp entering that stage. The entering brightness, in turn, is a well-defined function of the ratio of the D 1 stage clO 2 charge to the (EO) stage kappa number. In this study, we use the same model, together with the results of pulp- ing and bleaching experiments on southern U.S. red oak chips, to compare the bleachability characteristics of a con - ventional kappa number 15 pulp with that of an acid-prehydrolyzed pulp having, after 25% kappa number reduction by prehydrolysis, a similar kappa number. After the (EO) stage, the prehydrolyzed pulp had a lower extracted kappa number, but its brightness was lower than that of the control pulp. Both observations may be interpreted in terms of the relative amounts of residual lignin and hexenuronic acid present in the pulps. The full-sequence model described in our previous paper accurately described bleaching of the prehydrolyzed pulp. The model was used to evaluate the effects of the prehydrolysis on the optimized performance of the D 1 and D 2 stages. The brightness disadvantage of the prehydrolyzed pulp after the D 0 (EO) stages persisted over the lower range of D 1 charge multiples but diminished as the multiple was increased, eventually disappearing. The prehydrolyzed pulp required slightly less clO 2 than the control pulp when bleaching to high brightness in five stages. Application:�Bleach plant operators can use the findings of this study to improve operations by adjusting chemical charges. Additionally, those who have access to laboratory facilities can use the approach illustrated by this study to optimize bleaching processes by identifying the most advantageous distribution of chemicals among the various stages.