How do xylanase and hot acid stages differ at enhancing elemental chlorine-free bleaching of hardwood kraft pulp?

Abstract

Hardwood kraft pulp contains relatively high hexenuronic acid content which increases unnecessary consumption of bleaching chemicals and has negative effects on pulp brightness stability. Hexenuronic acid can be partly removed by an enzyme treatment or a hot acid treatment. The difference between enzyme and hot acid stages on removing hexenuronic acid and enhancing hardwood kraft pulp ECF bleaching has been investigated. A typical hot acid stage removed more than 60% of hexenuronic acid but only saved about 20% of chlorine dioxide to achieve 84% ISO, while xylanase treatment only removed 12% of hexenuronic acid but saved approximately 40% of chlorine dioxide to achieve the same brightness target. In addition to a slight reduction of hexenuronic acid, fiber ultrastructure analysis reveals that enzyme pretreatment removed hemicellulose layer which was originally present on the fiber surface. The removal of “barrier” xylan increases fiber accessibility to bleaching chemicals and decreases the diffusion resistance of delignification during bleaching.