Bleachability of alkaline pulps. Effect of quinones present in residual lignin

摘要

Unbleached chemical pulps produced by alkaline processes (kraft, NaOH, NaOH-Anthraquinone, IDE, Novacell) were analysed for their content in quinone groups. Residual lignins were extracted by mild acidolysis and analysed by ~(19)F NMR spectroscopy, after reaction with 1-(4-trifluoromethyl)-phenylhydrazine and by voltammetry. This latter method was performed on a glassy carbon electrode, by analysing the reduction wave of the quinone groups in water at pH 11.5 or in DMSO. This wave appeared in the -0.5 to -1.0V (vs SCE) range. The number of quinone groups measured by ~(19)F NMR varied from 0.02 to 0.07 per 200g of lignin. Kraft pulps were generally poorer in quinone groups than the other alkaline pulps. The ranking was the same with the two analytical techniques. Some differences existed also among pulps from the same process, depending on the operating conditions and the degree of delignification. Since ClO_2 is not very reactive with quinones, it is suggested that the bleachability differences between pulps upon ECF bleaching may arise from the variation of quinone groups in residual lignin.