In situ investigation of aggregate sizes formed using thermo-responsive polymers: Effect of temperature and shear

Temperature-responsive flocculants, such as poly(N-isopropylacrylamide) (PNIPAM), induce reversible particle aggregation upon heating above a lower critical solution temperature (LCST). The aim of this work is to investigate the aggregation of ground iron ore using PNIPAM and conventional polyacrylamide (PAM) flocculants in a continuously-sheared suspension, through in situ chord length measurements using Focused Beam Reflectance Measurement techniques and real-time imaging of the particle aggregates. In the presence of uncharged PNIPAM, particle aggregation occurs only upon heating to the LCST, and the aggregates continue to grow with further heating. Subsequent cooling re-disperses the aggregates, and repeated heating causes reformation. Unlike uncharged PNIPAM, anionic PNIPAM produces aggregates at temperatures below the LCST due to the polymer chains binding to two different particles via attractive interactions between the acrylic acid groups and the hematite surfaces, and can be added at temperatures above the LCST due to the formation of charge-stabilised micelles. Under continuous shear, the flocculant most able to resist aggregate size reduction was anionic PAM, followed by PAM, anionic PNIPAM, PNIPAM (6 MDa), and PNIPAM (122 kDa). Reversible aggregate breakage was found with all samples, except with PNIPAM (6 MDa) after being subjected to shear rates above 550 s⁻¹. Furthermore, heating of the PNIPAM-dosed suspensions at shear rates below 200 s⁻¹ produced larger and more breakage-resistant aggregates.