The impact of polyacrylamide flocculant solution age on flocculation performance

Aqueous solutions of high molecular weight polyacrylamides used to flocculate mineral slurries undergo time-based changes in their properties. Previous studies of the impact of ageing on flocculation performance have focused on time-scales of weeks or months, which has little relevance to industrial practice. In this study, ageing times from I h to 6 days were examined. Flocculation was achieved continuously in a Couette mixing device (stationary outer cylinder, rotating inner cylinder). The extent of aggregation was assessed from batch settling tests and in situ size characterisation with a focused beam reflectance measurement (FBRM) probe. The polyacrylamide dosages required to achieve measurable flocculation decreased as the flocculant age was increased, with optimal performance attained at 72 h. Flocculation using a 1-h-old flocculant solution consumed 75% more polymer than with an optimally prepared stock solution. The relationship between hindered settling rate and FBRM chord length measurements was found to be independent of flocculant age, but was sensitive to shifts in aggregate density caused by variations in mixing intensity. It is proposed that the early stages of flocculant dissolution involved the release of discrete polymer chains from highly agglomerated species, the former dominating flocculation activity. Optimal ageing maximised the discrete polymer concentration available for flocculation, leading to a significant increase in the aggregate size distribution but did not appear to impact upon the aggregate packing structure (density). (C) 2002 Elsevier Science B.V All rights reserved.