CAUSTIC EXTRACTION IN ELEMENTAL CHLORINE-FREE BLEACHING SEQUENCES FOR SOFT-WOOD KRAFT PULPS: A FUNDAMENTAL REVIEW AND ANALYSIS, 2008 Engineering, Pulping and Environmental Conference



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The alkaline extraction stage plays a pivotal role in the residual lignin removed from the pulp during Elemental Chlorine-Free (ECF) bleaching. The goal of this paper is to review and analyze the fundamental research work of alkaline extraction concerning ECF bleaching sequences. Comparisons will be made to past knowledge of extraction with chlorinated pulps, as well as how ECF extraction differs. Approximately 50 to 65% of the Klason lignin removed from softwood kraft pulps during a D0E sequence occurs within extraction. The delignification process is governed by physico-chemical phenomena, such as oxidized lignin solubility and lignin diffusion, as well as chemical modifications to the D0 oxidized lignin by oxygen and/or peroxide. Other physico-chemical processes, such as alkaline fiber swelling, do not significantly contribute to the lignin removal process. It is observed that the oxidative reinforcement of E stages with oxygen (EO), peroxide (EP) or their combinations (EOP) significantly lowers the post-extraction kappa number and increases the post-extraction brightness. Oxygen reinforcement does not afford a pulp that contains less lignin compared to an E stage. Oxidative reinforcement modifies the post-extraction residual lignin such that it consumes less ClO2 in the subsequent D1 stage to reach a given brightness target. The benefits of H2O2 reinforcement and elevated extraction temperatures (¡Ý90¡ãC) are not additive, indicating that competitive processes are involved. This may be caused by the boost in lignin diffusivity at temperatures >70¡ãC versus the H2O2 degradation of the lignin structure. An alternative explanation is the greater instability of H2O2 at elevated temperatures due to alkali-induced decomposition reactions.

Author: Brian N. Brogdon
CAUSTIC EXTRACTION IN ELEMENTAL CHLORINE-FREE BLEACHING SEQU
CAUSTIC EXTRACTION IN ELEMENTAL CHLORINE-FREE BLEACHING SEQUENCES FOR SOFT-WOOD KRAFT PULPS: A FUNDAMENTAL REVIEW AND ANALYSIS, 2008 Engineering, Pulping and Environmental Conference
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