Pre-Extraction of Hemicelluloses from Hardwood Chips Using an Alkaline Wood Pulping Solution Followed by Kraft Pulping of the Extracted Wood Chips

Abstract

Mixed southern hardwood chips were extracted with an alkaline wood pulping solution called kraft green liquor. This aqueous solution containing mainly sodium carbonate and sodium sulfide was applied at different alkali charges (expressed as Na2O) of 0, 2, 4, and 6% on dry wood weight. The extractions were performed at 160 °C for effective times ranging from about 1−2 h to determine the effect of extraction severity on pulp yield and composition of the extracted liquor. The severity of hemicellulose extraction time and alkaline charge controls the concentration of acetic acid and monosaccharide sugars available for downstream processing, the accumulation of degradation products such as organic acids and furans in the extract, and the pulp yield attainable for the extracted wood chips. As the alkali charge was increased, the amount of acetate side chains on the hemicelluloses and the dissolved lignin in the extract increased but the carbohydrate and sugars in the extract decreased appreciably. Water extraction (0% alkali addition) released the greatest amount of carbohydrates, up to 30 g/L measured as component sugars, but resulted in the greatest decrease in pulp yield, dropping from 47% to 35%. Extraction with 2% green liquor increased the pulp yield to 51% while greatly reducing the component sugars to 8 g/L. Data obtained in this work will allow selection of optimum hemicellulose extraction conditions for integrating the extraction operation into the Kraft pulping process.