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

• Published in: Tappi journal 2009-09, Vol.8 (8), p.10-18
• Main Authors: MCDONOUGH, THOMAS J., UNO, SHUNICHIRO, RUDIE, ALAN W., COURCHENE, CHARLES E.
• Format: Article
• Language: English
• ISSN: 0734-1415; 0734-1415
• DOI: 10.32964/TJ8.8.10

Earlier studies [1] have shown that when a hardwood kraft pulp is bleached in the D0(EO)D1ED2 sequence, the brightness of the pulp emerging from the D2 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 D1 stage ClO2 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 conventional 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 D1 and D2 stages. The brightness disadvantage of the prehydrolyzed pulp after the D0(EO) stages persisted over the lower range of D1 charge multiples but diminished as the multiple was increased, eventually disappearing. The prehydrolyzed pulp required slightly less ClO2 than the control pulp when bleaching to high brightness in five stages.