THE INFLUENCE OF THE BLEACHING MEDIUM ON CHLORINE DIOXIDE DELIGNIFICATION

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.

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