Flocculation of thermomechanical pulping spent liquor with polydiallyldimethylammonium chloride

Currently, the dissolved lignocelluloses in the spent liquor (SL) of a thermomechanical pulping process are treated in wastewater treatment systems and thus they are wasted. In this work, polydiallyldimethylammonium chloride (PDADMAC), with different molecular weights, was used for flocculating lignocelluloses of SL and thus isolating them from SL. Results showed that the maximum removals were 38% via treating SL with 100 mg/L of PDADMAC (with 1045 kg/mol molecular weight) at 25 °C for 30 min. The focused beam reflectance measurement of the flocculation process revealed that the chord length of the flocs with the maximum square weighted counts was increased from 70 to 100 μm and also their maximum square weighted counts was increased from 5 to 25 μm²/s. The flocs contained 60.71-74.41 wt% PDADMAC, the balance of lignocelluloses and the heating value of 24-25 MJ/kg. The high molecular PDADMAC generated flocs with more organics and a higher heating value.