Evolving a flocculation process for isolating lignosulfonate from solution.

• PDADMAC was used as flocculant for lignosulfonate of pulping spent liquor. • The molecular weight of PDADMAC substantially affected the flocculation process. • Floc formation and sedimentation were studied in stirring and non-stirring systems. • The impact of salt and sugar on floc formation was studied. • The results showed promising approach for developing a cleaner pulping process. Lignosulfonate is a major component of the spent liquor of the sulfite pulping process. Flocculation has been used worldwide for isolating colloidal substances from solutions and thus can be adapted for separating lignosulfonate from the spent liquor. The adaptation of flocculation process would generate lignosulfonate-based flocs with a potential use as a value-added product and reduce the concentration of lignosulfonate in the spent liquor implying that this process can facilitate spent liquor's treatment operation. To evaluate the performance of this flocculation process, the interaction of lignosulfonate with polydiallyldimethylammonium chloride (PDADMAC) having various molecular weights was fundamentally studied in simulated systems under stirring and non-stirring environments. Focused beam reflectance measurement (FBRM) and vertical scan analyzer confirmed the fast kinetics of floc formation. A correlation was observed between the flocculation index of complexes and the molecular weight of PDADMAC. Increasing the PDADMAC's molecular weight led to the production of more flocs with larger sizes in the stirred system. However, larger flocs were weaker than smaller ones. Sedimentation analysis proved that the higher molecular weight PDADMAC generated more flocs with looser structures, but these flocs settled faster than the flocs produced by PDADMAC with smaller sizes. The results confirmed that the non-stirred system was more sensitive than stirring one to the dosage of PDADMAC. The flocculation efficiency was improved in the presence of salt while no change in the flocculation process was observed in the presence of sugar. The outcomes of this study suggest a promising alternative to isolate lignosulfoante from the spent liquor. [ABSTRACT FROM AUTHOR]