Some aspects on TCF-bleachability of softwood alkaline pulps

Abstract: The bleachability of softwood oxygen delignified pulps hasbeen studied, i.e. the ease (requirement of bleachingchemicals) with which the pulps can be brightened to a targetbrightness in totally chlorine free (TCF) bleaching stages,including hydrogen peroxide, ozone and chelating agent stages.Different cooking processes have been compared and theinfluence of different pre- and post-treatments on the kraftprocess has been investigated. The influence of differentcooking parameters in the kraft cook on the bleachability hasalso been studied, as well as the influence on thebleachability of the kappa number of the pulp after the cookand after the oxygen stage.Pulps produced by alkaline sulfite processes, ASAM andMSSAQ, showed better bleachability and process selectivity(viscosity at a given brightness) than pulps produced by themodified kraft process. The bleachability of the modified kraftpulps could be improved by a postsulfonation.The bleachability was improved by terminating the cook at ahigher kappa number level, when oxygen delignifying to the samekappa number level before bleaching. Improvements are alsoachieved by starting the oxygen stage from the same kappanumber and extending the oxygen delignification to a lowerkappa number level. The process selectivity and the yield areimproved in the same way.The QPQP'-bleachability (P'=peroxide stage with the additionof magnesium ions) was improved by changing the cookingconditions in a kraft cook leading to a shorter cooking time,i.e. by increasing the hydroxide ion concentration, thehydrogen sulfide ion concentration, or the cooking temperatureor by decreasing the sodium ion concentration. Exceptions couldbe seen for very high [HO-], where the bleachability even deteriorated, andwhen the temperature was increased at very high chemicalcharges, where no more improvement was achieved.The pulp with the best QPQP'-bleachability in a series ofpulps had a lower light absorption coefficient (k)/lignin content (kappa number corrected for thehexenuronic acid contribution (kappa'')) ratio already afterthe cook as well as after the oxygen stage and a higher?-O-4 content after the cook. A decreasedk/kappa'' ratio after the cook is most probably due toless redeposition of dark dissolved organic material from theblack liquor when the residual hydroxide ion concentration isincreased. The degree of delignification in a subsequent oxygenstage becomes lower for the pulp with a lowerk/kappa'' ratio after the cook and the metal ion contentin the pulps was lower. Additives like, for example sulfitereduce the redeposition of lignin and lead to a brighterpulp.The changes leading to improved bleachability for a pulp arenot always followed by improved yield and process selectivity.The hydrogen sulfide ion concentration and the sodium ionconcentration influenced the yield and selectivity positivelyin the same way as the bleachability. Increased hydroxide ionconcentration and temperature, however, lead to lower yield andpoorer selectivity. Thus for an optimal product a compromisebetween a good bleachability and high yield and good strengthproperties must be chosen.Keywords: Bleachability, chlorine-free bleaching,softwoods, kraft pulps, alkaline sulfite pulps, oxygen,hydrogen peroxide, ozone, hydrogen sulfide ion, hydroxide ion,temperature, sodium ion.