Near-neutral pre-extraction of hemicelluloses and subsequent kraft pulping of southern mixed hardwoods

Abstract

Southern mixed hardwood chips were extracted with alkaline solutions at different chemical charges, times (45-110 min), and temperatures (125°C-160°C). At high alkali charges (10% and 20% sodium hydroxide [NaOH] as sodium oxide [Na2O]), the extract was strongly alkaline (pH about 13) and 17%-40% of the wood was dissolved. Subsequent kraft cooking of the extracted chips yielded 5%-7% less pulp than that of control kraft pulps. However, at reduced alkali charge, just sufficient to approximately neutralize the acids released during pre-extraction, the pulp yield (on original wood) after subsequent kraft pulping was not affected. In this case, about 5%-10% of the wood substance is removed during pre-extraction with 3% NaOH or 3% green liquor (+0.05% anthraquinone) at 140°C and 160°C for 60, 90, and 110 min. The green liquor extract obtained after 110 min at 160°C contained 2.1% (oven-dry weight basis) of sugars, 2.1% acetic acid, and 1.6% lignin accounting for 64% of the wood weight loss. Kraft pulping of the pre-extracted wood chips performed at 12% effective alkali charge showed significantly improved delignification rates and approximately the same or slightly higher yield than the kraft control at 15% effective alkali. The near-neutral green liquor+anthraquinone pre-extraction kraft pulps showed lower refining response but higher tear resistance and similar tensile strength compared to control kraft pulps.