Extended oxygen delignification. Part I: effects of multi-stage oxygen bleaching on softwood kraft pulp and residual lignin. Comparison with chlorine dioxide delignification

摘要

A softwood kraft pulp was submitted to successive oxygen stages (O, OO, OOO). Residual lignin after treatment was extracted by acid hydrolysis and characterised by ~(13)C NMR and GPC. The progressive decrease in the efficiency of the O treatment in a multiple O sequence was explained by the poor reactivity of some of the free phenolic lignin structures with oxygen. Oxygen (O) and chlorine dioxide (D) delignification were compared. Contrary to 000..., the DEDEDE... process led to total delignification. Both chemicals attacked the free phenolic groups in lignin as evidenced by the important decrease in their number measured after treatment. However, their disappearance was more complete in the case of CIO_2. Moreover a higher number of COOH groups were formed in the latter case. According to mechanisms given for chlorine dioxide delignification, hypochlor-ous acid (CIOH) is produced as by-product during a D stage and would react with residual lignin. This would explain the better efficiency of D compared to O stages.