Aggregation and Sedimentation Performance of Lignin and Hemicellulose Derived Flocs in the Spent Liquor of Thermomechanical Pulping Process

Abstract

Spent liquors produced in the pulping industry contain dissolved lignocellulosic macromolecular components that are currently burned or treated in wastewater treatment systems. However, they can be isolated by flocculation to make added-value products. In this work, lignin and hemicelluloses were extracted from spent liquor (SL) of a thermomechanical pulping process (TMP) and treated by polydiallyldimethylammonium chloride (PDADMAC). The flocculation of lignocellulosic compounds of SL with three different molecular weights of PDADMAC was studied comprehensively in this work. The settlement of the generated flocs was also investigated by means of advanced techniques, and the results were related to the characteristics of PDADMAC and lignocellulosic compounds. The maximum removals of lignin (57%) and hemicellulose (36%) were achieved by larger PDADMAC with a molecular weight of 1045 kg/mol at the dosages of 100 mg/L and 80 mg/L, respectively. The properties of lignin and hemicelluloses, as well as zeta potential and focused beam reflectance measurement (FBRM) analyses, confirmed that the flocculation of lignin was mainly based on charge interaction, while that of hemicellulose was based on bridging mechanism. PDADMAC/pure lignin system showed a faster sedimentation rate with more compact flocs than both PDADMAC/pure hemicellulose and PDADMAC/industrial SL (TMP) systems. The PDADMAC/hemicellulose system had flocs with the loosest structure and poorest settling performance. The results of this study provide a means to extract and use lignocellulosic compounds of spent liquor via selective flocculation and separation strategies. Lignin/PDADMAC flocs and hemicellulose/PDADMAC flocs can be separated from their solutions by sedimentation and filtration, respectively.