Mild alkaline hot-water extraction of lignin from spruce wood

The aim of this study was to isolate lignin from spruce wood with an Accelerated Solvent Extractor (ASE) using low concentration of aqueous NaOH and characterise the products in order to understand how the process parameters affects the chemistry involved. Two concentration of alkali were studied – 1% and 2% aqueous NaOH. Extraction was performed for 4 hours total time, but different extraction sequences were studied: 1×240 min, 2×120 min, 4×60 min, 6×40 min, and 12×20 min. It was found that higher alkali concentration allows extracting more lignin from spruce wood. The extraction sequence has great influence on the process – shorter extraction times extracted up to 10 times more lignin compared to longer extraction times. Xylose and galactose were the most abundant carbohydrate impurities in the isolated lignins. The average molar mass of lignin increased with the extraction time and concentration of alkali.