Advances in Understanding the Basics of the First Alkaline Extraction Stage in Bleaching, 1997 Pulping Conference Proceedings

Recent proposed changes in the environmental performance of kraft bleaching operations have focused extensive research on new bleaching agents and improvements in chlorine dioxide. Less attention has been directed towards the subsequent alkaline extraction stage. Although researchers have identified the optimal charge of NaOH to promote delignification and AOX reduction during an E-stage, little is known of the complex reactions involved in this process.

This paper explores the chemistry involved in a conventional E, E+O, and E+O+P stage after the initial D stage. Employing advanced spectroscopic techniques, we have demonstrated that the concentration of carboxylic acid groups generated in an E and E+O are comparable, suggesting oxygen does not significantly enhance carboxylic acid formation. Hydrolysis of methyl ester structures formed during the preceding ClO2 stage appears to be the main reaction causing the formation of carboxylic acid groups during the alkaline extraction. Although phenolic groups were found to be depleted during the alkaline extraction, this reaction was not found to significantly increase the carboxylic acid content. Bleaching studies have shown the chemistry of the alkaline extraction stage to influence subsequent chlorine dioxide brightening reactions employing D(E*); E*= E, E+O, E+O+P.