Towards an accurate determination of lignin in chemical pulps : The meaning of kappa number as a tool for analysis of oxidable groups

Abstract: Lignin content is the most important parameter in chemicalpulping. At present, kappa number is the most common parameterfor determination of lignin content and the base for otheron-line methods and for industrial automation (Chapter 1).However, a substantial portion of the kappa number originatesfrom non-lignin structures, for example from pulp xylan, whichhas been severely degraded and modified during the kraft cook(Chapter 2).During a systematic research about the hexenuronic acid(HUA) structure, one of the most important structures derivedfrom carbohydrates, a new, simple, fast and sensitivecolorimetric method has been developed for the quantificationof the HUA group (Chapter 3). Furthermore, it has beendemonstrated that the quantitative contribution of the HUAgroups to the kappa number is 0.84-0.86 kappa units per 10µmol of HUA (Chapter 4).In systematic work about the reactions involved in the kappanumber determination, it was found that an addition ofpermanganate onto all the double bonds in the aromatic rings oflignin takes place, resulting in a reduction of permanganate toMnO2. The present standard method should therefore beimproved by rejecting or modifying the iodometric titrationmethod, correcting the concept of remaining permanganate,resetting the applicable region of permanganate consumption andshortening the reaction time (Chapter 5).The quantitative fundament for the kappa numberdetermination has been found to be a consumption of 11-12equivalents of KMnO4per mole of C9units from any wood lignin structure. Besides HUA,any other non-lignin but oxidizable structure will, more orless, influence the accuracy of the kappa number determination;the most noticeable structures being double bonds, aldehydes or?-keto carboxylic acids (Chapter 6).Based on the above fundamental research, anoxymercuration-demercuration-kappa (Ox - Dem kappa) numberdetermination has been developed. All the noticeablyinterfering non-lignin structures are eliminated by one Ox -Dem procedure before a conventional kappa number determinationso that a more accurate determination of aromatic (Klason)lignin content in pulps then can be easily, quickly anddirectly obtained (Chapter 7).For industrial and laboratory made unbleached chemicalpulps, it has been revealed that for different pine kraft orsoda pulps 2.9 - 4.8 kappa units were from non-ligninstructures, corresponding to 13-26% of the observed kappanumber. For birch kraft pulps, these data are much higher:8.2-9.4 units, 60 - 80 % of the observed kappa number! Amongthese non-lignin contributions, approximately 30-60% are causedby HUA structure while the others are from double bonds,aldehydes or ?-keto carboxylicacid structures, except forpine soda pulps where the non-lignin contributions are mainlyfrom the latter types of structures. Preliminary application ofthe Ox - Dem kappa number has resulted in brand newdelignification curves for kraft cooking and OQP bleaching ofbirch pulp. A possible reformation of "lignin" in kraft cookingand a higher bleach-ability after low alkalinity and lowtemperature cooking have been noticed (Chapter 8).The easy and more accurate determination of lignin contentdescribed in this thesis suggests a promising future for bettercontrol of cooking and bleaching and for further optimizationand modification of present chemical pulping procedures.