Role of Temperature-Sensitive Polymers in Solids Dewatering
Efficient solids dewatering (especially fine solids) depends on rapid particle settling and sediment water release. High molecular weight polyacrylamide (PAM) type polymers are widely used as flocculants for that purpose. They are effective in promoting solids settling but not very good in water release. The investigation of a novel process aid, poly(N-isopropyl acrylamide) (PNIPAM), was carried out to improve dewatering. Four PNIPAM polymers with various molecular weights from 25,000 up to 10 million Da were tested to treat silica suspensions. Primary results showed that this type of polymer enhanced both solid settling and sediment water release when the temperature was cycled between room temperature and 50 oC. The polymer performance is strongly dependent on polymer molecular weight and solution ionic strength. The higher the molecular weight, the lower the polymer dosage required to induce an effective solid aggregation, good water clarity and good water release. All these benefits of PNIAPMs are related to the different action mechanism of PNIPAM polymer from that of PAM polymer. It was found that the polymer phase transition between hydrophilic and hydrophobic is the key to making them work in this manner. At higher temperature, the polymer induces solids aggregation via a hydrophobic attractive force and afterwards when the temperature cools down, the polymer becomes hydrophilic and makes the aggregates become dispersed. In addition, since the polymer critical solution temperature (CST) is not too high (~32 oC), it is practical for their industrial application.