Fundamentals of High Pressure Oxygen and Low Pressure Oxygen-Peroxide (Eop) Delignification of Softwood and Hardwood Kraft Pulps: A Comparison, 1996 Pulping Conference Proceedings

YuJun sun Dimitris & S. Argyropoulos

McGill University

In an effort to elucidate and compare the fundamental changes of softwood and hardwood kraft lignins during high pressure oxygen and low pressure oxygen-peroxide delignification (Eop) stages, two conventional kraft pulps were prepared from such species and accordingly delignified using similar conditions. Residual lignins from the untreated kraft pulps, the oxygen and the Eop delignified pulps as well as the lignins dissolved in their respective liquors were isolated and examined by detailed analyses and quantitative 31 P NMR spectroscopy. Quantitative 31 P NMR using 2-chloro-4,4,5,5- tetramethyl- 1,3,2-dioxaphospholane, as the phosphitylating reagent, allowed the determination of uncondensed and condensed phenolic units in these technical lignins. The results of this technique adequately compared to those of oxidative degradation. The formation of carboxylic acids and the degradation of condensed phenolic structures were found to be the main pathways which may account for lignin dissolution during the oxygen or the Eop delignification stages. Since significant degradation of the uncondensed and condensed phenolic units accompanied the oxygen and the Eop stages, the formation of carboxylic acids most likely compensate for the elimination of phenolic hydroxyl groups by keeping the lignin fragments hydrophillic enough for dissolution in alkaline media. The uncondensed phenolic units of hardwood kraft residual lignin reacted more efficiently than their softwood counterparts during both the oxygen and the Eop stages. The chemical and physicochemical considerations that affect the efficiency of an oxygen and an Eop stage are similar. However, due to the milder nature of the Eop stage, the residual lignins resulting from it are less affected than their high pressure oxygen counterparts.