Examining the effect of conditioning sequence of polymer flocculants and coagulants on sedimentation and filtration of oil sands tailings

• Adding cationic coagulants prior to nonionic flocculants improved slurry filtration. • Adding anionic flocculants prior to cationic coagulants improved slurry filtration. • Reversing the sequence of addition in the above two cases made filtration ineffective. This study investigated the impact of conditioning sequence of polymer flocculants and coagulants on the sedimentation and filtration of oil sands mature fine tailings (MFT). Nonionic poly(ethylene oxide) (PEO) or anionic polyacrylamide (A3370) was used as the flocculant, and cationic polydiallyldimethylammonium chloride (polyDADMAC) was used as the coagulant. Irrespective of addition sequence, the combination of flocculant and coagulant yielded higher initial settling rate and clearer supernatant than when either was used alone. However, the filtration behavior was found to depend significantly on dosing sequence of the flocculant and coagulant. A much faster filtration rate was observed when the MFT was first treated by the cationic coagulant polyDADMAC followed by the nonionic polymer PEO. In contrast, adding the anionic flocculant A3370 prior to the cationic coagulant polyDADMAC resulted in a much faster filtration rate. Floc size distribution and floc surface charge were measured to understand the effects of conditioning sequence on the sedimentation and filtration performance of MFT. It was found that the two optimal addition sequences led to larger and more porous flocs whose surface charges were closer to zero than the flocs obtained when the addition sequence was reversed. These findings underscore the importance of optimizing the sequence of flocculant and coagulant addition to enhance solid–liquid separation, with implications extending beyond oil sands tailings treatment.