Weakly protonated polyethylenimine induces SiC flocculation in alkaline suspensions

Silicon carbide (SiC) suspensions exhibit high colloidal stability under alkaline conditions due to their strong negative surface charge (zeta potential <-30 mV), posing significant challenges for solid-liquid separation in industrial processes. Conventional acid-washing methods suffer from equipment corrosion, ionic contamination, and environmental risks. Herein, we propose polyethylenimine (PEI) with a low molecular weight (M_w = 600) and a weak protonation degree of 8.68-16.97% at pH 10 as a green and efficient flocculant for rapid sedimentation of alkaline SiC suspensions (pH 8-12). Systematic investigations were conducted to evaluate the effects of PEI dosage, pH, and solid content on zeta potential, sedimentation kinetics, and supernatant clarity. At an optimal PEI dosage of 0.2-0.4 g L^-1, supernatant clarity exceeded 98.63%. Flocculation kinetics were quantitatively analyzed using aggregation and collision models, revealing that PEI-induced sedimentation is governed by a synergistic mechanism involving residual electrostatic attraction, hydrogen bonding, and polymer bridging under alkaline conditions-distinct from the charge neutralization-dominated process in acidic environments. Notably, this kinetic analysis also explains the dosage-dependent sediment morphology: 0.2 g L^-1 yields dense flocs due to balanced bridging and low breakage rate, while 0.4 g L^-1 produces looser aggregates due to steric hindrance from excess polymer chains. Compared to conventional flocculants such as polyaluminum chloride (PAC) and cationic polyacrylamide (CPAM), PEI demonstrated superior sedimentation performance. This work provides a scalable, cost-effective, and environmentally benign strategy for efficient solid-liquid separation of alkaline SiC suspensions, with promising applications in fine ceramic powder recovery and wastewater treatment.