Optimization of ECF bleaching of kraft pulp: Part 1. Optimal bleaching of hardwood pulps made with different alkali charges

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 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 fac-tor is the most economical.