Alkaline extraction of hemicelluloses from aspen chips and its impact on subsequent kraft pulping

A mild alkaline treatment was applied to aspen chips prior to Kraft pulping in order to extract hemicelluloses for utilization as biofuels. The extraction was performed at 50-90 °C, hence not requiring costly pressurized reaction vessels. It resulted in a recovery of 40-50 kg of hemicellulose per tonne of chips, and gave same pulp yield after subsequent pulping of the extracted chips residue as compared to a control cook. The pre-extraction process requires careful optimization of the pulping process, and allows shorter cooking time and lower chemical charges. Kraft pulps obtained from pre-extracted chips have a slightly higher cellulose/hemicellulose ratio, and demonstrate a small decrease in tensile index (∼10%), but an improved brightness and shive content. While the hemicellulose yield for this process is low, this problem is outweighed by the fact that pulp properties and pulping yield can be maintained at a high level. The recovered hemicellulose can be concentrated/isolated more easily than furfural-containing smaller fragments obtained by autohydrolysis or dilute-acid treatment.