Effect of polyacrylic acid molecular weight on the floc stability during prolonged settling

Settling tests are normally carried out for short durations to study dewatering characteristics of materials. Natural gravitational settling and associated consolidation processes continue for longer periods before a substantially stable structure is attained. For example, the structural stabilization of a tailing dam involving fine particulates continues for years. To simulate the conditions of gravitational stabilization of particulate materials in the presence of polymers, in this investigation batch settling trials were carried out for prolonged durations with the model colloidal alumina and polyacrylic acid (PAA) flocculants of different molecular weights. It is observed that a critical molecular weight exists where superior flocculation and consolidation performances are observed due to synergistic effects of charge neutralization and bridging. Based on the interesting findings, it is suggested that the design of an optimum dewatering reagent scheme need not be based on short duration sedimentation and/or filtration trials alone. The long-term gravitational stabilization behaviour is an important area of concern from environmental and safety points of view and should be appropriately addressed.