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The chemistry of ozone bleaching of chemical pulps was explored by ozonating residual lignins isolated from conventional kraft and SuperBatch pulps in methanol/water medium to detect possible differences in the reactivity of the two types of lignin. SuperBatch lignin was ozonated also in water to study the effect of ozonation medium on the lignin reactivity. The residual lignins were found to display similar reactivities in methanol/water, implying that ozonation should result in equal delignification rates for both conventional kraft and SuperBatch pulps unless the rates of reagent diffusion in the pulps are different. The lignins were partly oxidized to volatile and nonvolatile low-molecular weight oxidation products by the so-called “peeling mechanism”, according to which oxidation products go into solution and insoluble reaction products resemble the starting lignins. The reaction products obtained upon ozonation of SuperBatch lignin in neutral water resembled those formed in methanol/water, but their yield was much lower. This is probably due to the better solubility of lignin in methanol/water than in water and/or higher degradability of lignin by ozone than by radicals formed as ozone decomposition products.

Ozonation of conventional kraft and SuperBatch residual lignins in methanol/water and water