Optimization of ECF Bleaching of Kraft Pulp: I. Optimal Bleaching of Hardwood Pulp Made With Different Alkali Charges, 2006 Engineering, Pulping, & Environmental Conference

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In an earlier modeling study, we showed that when a hardwood kraft pulp is bleached in the D₀(EO)D₁ED₂ sequence with a D0 stage kappa factor of 0.20, 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 results of pulping and bleaching experiments on southern U.S. red oak chips to extend the model to account for the effect of changing the D₀ kappa factor and use it to assess the effect of digester alkali charge on pulp bleachability. We also show how this approach can be used to minimize the total ClO₂ requirement for any desired final brightness and to optimize the allocation of ClO₂ to the three D stages of the D(EO)DED sequence or the two D stages of the D(EO)D sequence. Among the conclusions resulting from application of the model are the findings that the combination of high effective alkali and high D₀ kappa factor gives pulp with a higher final brightness ceiling than any other combination but that, for brightness targets of 91 or lower, the combination of low effective alkali and low kappa factor is the most economical.