Influence of ionic strength on kraft cooking and subsequent TCF-bleaching

The TCF-bleachability of softwood kraft pulps produced with different ionic strength profiles has been investigated. In addition, the relationship bet ween the bleaching response and some of the chemical structures in the pulp were studied. The ionic strength was changed by adding NaCl(s) at two alte rnative positions in the cook: after the pretreatment, or in the final part of the cook to give three different process variants. All cooks began with a standardized hydrogen sulfide ion pretreatment stage at low alkalinity. Pulps at two different kappa number levels, about 21 and 16, were investiga ted. After further delignification in an oxygen stage, the bleachability wa s studied with a QPQP*-sequence (Q=chelating agent stage, P=peroxide-stage, *=Mg ions addition). Pulping with a higher ionic strength led to a lower delignification cation rate and thus a greater hydroxide ion consumption to reach a given kappa nu mber. The results also show an increasing light absorption coefficient/kapp a number ratio for unbleached pulps with increasing ionic strength in the c ook. The degree of delignification in the oxygen stage was higher with a hi gher ionic strength in the cook. The bleachability as well as the yield and the process selectivity were improved by a low ionic strength in the cook for all the three investigated process variants.