The link between kraft cooking and bleaching : influence on ECF bleachabiblity and bleaching effluent parameters

Abstract: The link between kraft cooking and bleaching was investigated, with the focus on how cooking kappa number, cooking conditions and pre-bleaching stages influence ECF bleachability and environmental parameters in the bleaching effluent.



In one part of the study standard cooking conditions were used to produce birch and spruce pulps of different kappa numbers and then we studied how the AOX in the ECF bleaching effluent was affected by oxygen delignification, and hot acid stages. The amount of unstable AOX (eliminated during storage at pH 7 and 35¡C) in the D0-effluent was dependent on the amount of HexA (hexenuronic acid) removed from the pulp in that stage. This was evidence that the unstable AOX originated from HexA. The amount of stable AOX (not eliminated during storage) was related to the lignin content of the pulp. With standard cooking conditions and spruce as raw-material, the effects of kappa number and oxygen delignification on the COD levels in D(EO)D(EP)D and a Q(PO)TP effluents were studied. The COD level prior to effluent treatment was higher for the TCF than for the ECF effluent. Since the TCF effluent had lower lignin content and considering the limited biodegradability of lignin, our results indicate that the TCF effluent will have a lower level than the ECF effluent after biological treatment.



In an other part of the study we included variations in the cooking conditions and looked at the effects on the brightness development during ECF bleaching. For both spruce and birch, a higher hydroxide ion concentration in the cook gave a brighter unbleached pulp and it also led to a higher final ECF bleached brightness for birch pulps. For spruce pulps, a higher cooking alkali only gave better bleachability at low cooking kappa number and when no oxygen delignification was used. In the case of birch, a higher hydrogen sulphide ion level during cooking did not affect the unbleached brightness but it gave a better brightness gain in the ECF sequences, particularly when the hydroxide ion concentration was low. These bleachability differences affected the brightness of the pulp but they did not affect the kappa number reduction in the ECF sequence. Brightness reversion in humid and dry atmospheres was included in the birch bleachability study. The humid brightness reversion correlated with the HexA content after bleaching, but dry brightness reversion did not correlate with humid brightness reversion or directly with any of the investigated parameters