Optimization of ECF Bleaching of Kraft Pulp: II Effects of Acid Prehydrolysis on Hardwood Pulp Bleachability, 2007 Engineering, Pulping & Environmental Conference

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In earlier studies, we showed 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 the present study, we use the same model, together with the results of pulping 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. Bleaching of the prehydrolyzed pulp was accurately described by a full-sequence model described in earlier reports. 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.