EXTENDED OXYGEN DELIGNIFICATION OF HIGH- YIELD KRAFT PULP. CORRELATION BETWEEN RE- SIDUAL LIGNIN STRUCTURES AND BLEACHABILI- TY BY OXYGEN AND CHLORINE DIOXIDE

A promising method for maximizing the yield of kraft pulp seems to be cooking to kappa numbers between 40 and 45 with the addition of polysulfide and anthraquinone (PS/AQ) to the kraft cooking liquor, followed by oxygen delignification to kap-pa numbers around 22 and subsequent bleaching with more spe-cific bleach chemicals. The bleachability of the oxygen delignified high-yield PS/AQ kraft pulp and of conventional and ITC-type kraft pulps of comparable kappa numbers is in-vestigated and correlated to structural parameters of the lignin as determined by NMR spectroscopy. Initial bleachability (kap-pa number reduction in the O and D0 stage) of the high-yield PS/AQ pulp is not affected by the apparent low content of free phenolic hydroxyl groups in the pulp residual lignin. Bleach-ability of unbleached pulps correlates to the content of lignin in the pulp. The attempt to correlate pulp bleachability to single lignin structural parameters as determined by NMR spectrosco-py does work when comparing pulps which have been subjected to chemically similar conditions during pulping, like kraft pulps cooked using the same white liquor. However, when rather dis-similar chemical conditions (e.g. the addition of PS/AQ) are used during the pulping, it is not possible to look at one single structural property of the lignin and correlate this to pulp bleachability. Apparently, PS/AQ pulp is easily delignified by either oxygen or chlorine dioxide in spite of an apparent low level of free phenolic hydroxyl groups