Lignin precipitation on the pulp fibers in the ethanol-based organosolv pulping

Ethanol pulping of eucalyptus wood chips and wheat straw was conducted to investigate the precipitation of dissolved lignin back onto the fibers in pulping and washing. Results show that a large amount of lignin particles are present on the fibers after pulping and each stage of washing in a four-stage washing sequence, which was identified by both light microscopy and SEM, and by XPS. When the ethanol concentration of a lignin ethanol solution is reduced or the temperature drops as in the pulp washing process, lignin precipitation occurs. It has been concluded that high kappa number of ethanol pulp is mainly due to the precipitation of the dissolved lignin from the pulping liquor, which accounts for the easy bleachability of the ethanol pulp.