Solid-liquid separations with a temperature-responsive polymeric flocculant: effect of temperature and molecular...

The effects of temperature and molecular weight of the temperature-responsive polymer, poly(N-isopropylacrylamide) (PNIPAM) were investigated in the solid-liquid separation of silica and alumina mineral particles. Suspensions dosed with PNIPAM at 25 degrees C were stable and did not settle. When the temperature was raised above the polymer lower critical solution temperature (LCST) (>32 degrees C), the suspensions were found to have high settling rates, large particle aggregate sizes and high suspension shear yield stresses (tau(y)). The sediment bed solids volume fraction (phi(f)), of these suspensions was found to increase after a temperature decrease below the polymer LCST and was attributed to a decrease in the attractive particle-particle interactions as shown by a corresponding decrease in shear yield stress, with decreasing temperature. Settling rates were found to increase with molecular weight when suspensions were dosed at 25 degrees C and settled at 50 degrees C. Increasing polymer molecular weight resulted in increased molecular polymer adsorption at 25 degrees C. Greater initial adsorbed amounts of polymer on the surface produced more nucleation sites for deposition of additional polymer as the temperature was increased from 25 degrees C to above the LCST where polymer phase separation occurred. When the polymer was dosed at 50 degrees C, the rate of sedimentation was very low. Under these conditions, the polymer molecules associate with each other to form polymer aggregates of typically 1250 nm diameter. These colloidal polymer aggregates do not readily deposit on the particles surfaces such that mineral particle aggregation does not readily occur.