Towards sustainable pulping of bagasse: silica migration, hexenuronic acid removal and ultrafiltration-based spent liquor utilization

Abstract

Transforming the utilization of agricultural residues from single-purpose papermaking into high-value biorefinery strategies can unlock their full potential. The industrial valorization of bagasse is currently hampered by the inefficiency in bleaching chemical use and the limitations of alkali recovery out of silica-rich spent liquors. To address these challenges, this study establishes an integrated biorefinery strategy combining acid hydrolysis pretreatment (A-stage) for reducing usage of bleaching chemicals along with membrane-based fractionation of spent liquor. In the fiber line, incorporating an A-stage prior to elemental chlorine-free (ECF) bleaching selectively removed half of hexenuronic acid (HexA). This targeted elimination reduced chlorine dioxide (ClO2) consumption by 25% despite an expected compromise in losing viscosity. Possible migration (fate) of silica from its free form SiO2 at bagasse to form of calcium silicate (CaSiO3) or calcium-sodium silicate (Na2CaSiO4) at the pulp fiber is discovered. Ultrafiltration utilizing a 0.5 kDa-sized membrane retained high-molecular-weight ligninxylan complex fractions in the retentate while allowing the permeation of 72%-recoverable hydroxy acids (enriched with 2-hydroxybutanoic acid and glucoisosaccharinic acid) from permeate, opening up the promise of using a tailored ultrafiltration separation in recovery of alkali and hydroxy acids instead of costly recovery boiler for bagasse mill.