THE INFLUENCE OF THE BLEACHING MEDIUM ON CHLORINE DIOXIDE DELIGNIFICATION

Abstract

Chlorine dioxide delignification (D₀) stages performed in 90% ethanol resulted in post-D₀ kappa numbers that were 2 to 2.5 units lower than those performed in water. In spite of the reduction in kappa number, various gravimetric and UV spectroscopic techniques for measuring residual lignin indicated that the ethanol medium did not significantly enhance D₀ stage lignin removal. Differences in post-D₀ kappa numbers between the ethanol-based and aqueous systems were ascribed to how the bleaching medium affects ClO₂ oxidation of residual lignin. Ethanol-based D₀ stages resulted in an oxidized lignin that contains fewer muconic acid structures and more quinone structures than the aqueous-based systems. Indirect evidence from sodium hydrosulfite reduction implied that quinone moieties consume less KMnO₄ than aromatic structures in lignin during the kappa number test.

ACKNOWLEDGMENTS

The authors thank Drs. E.W. Malcolm, W.J. Frederick, and A.J. Ragauskas for their critical review of this work, and the Institute of Paper Science and Technology and its Member Companies for their financial support. Portions of this work were used by B.N. Brogdon as partial fulfillment of the requirements for the Ph.D. degree at the Institute of Paper Science and Technology. Parts of this research work were presented at the Fifth European Workshop of Lignocellulose and Pulp on August 30 September 2, 1998in Aveiro, Portugal.