Effect of molecular weight of poly(N‐isopropyl acrylamide) temperature‐sensitive flocculants on dewatering

The influence of molecular weight (MW) and dose of Poly(N‐isopropyl acrylamide) (PNIPAM) (temperature‐sensitive flocculant) on sedimentation rate, sediment density, and supernatant clarity of silica suspensions was investigated. The addition of PNIPAM resulted in rapid sedimentation (T > critical solution temperature, CST) and low sediment moisture (T < CST). Higher MW polymers resulted in more effective flocculation and sediment consolidation. At 10 ppm, PNIPAM (3.6 million Da) produced 20 m/h settling rate and 48 vol % solids sediment density, whereas 0.23 million Da polymer produced 0.1 m/h settling rate. PNIPAM produces effective flocculation and consolidation by cycling the interparticle interactions between repulsion and attraction as temperature is cycled around the CST. The change in temperature produces a hydrophilic/hydrophobic transition of the polymer, influencing adsorption onto the surface and the inter‐particle forces. Conventional polyacrylamide flocculants (not influenced by temperature), cannot be used to produce both rapid sedimentation and dense sediments. © 2009 American Institute of Chemical Engineers AIChE J, 2009