Synergy between Dual Polymers and Sand-to-Fines Ratio for Enhanced Flocculation of Oil Sand Mature Fine Tailings

The current challenges in dewatering oil sand mature fine tailings (MFTs) by flocculation using a single flocculant include the requirement of very high doses and high cost, inefficient flocculation, and low content of solids in sediment. The obtained flocculated sediment with less than <40 wt % solids is difficult in further consolidation and dewatering for the final disposal. In this work, we examined a new approach for dewatering by dual polymer flocculation in combination with varying sand-to-fines ratios (SFRs) in MFTs. The dual polymers were an anionic polyacrylamide (APAM), and a nanosized cationic hybrid polymer (NHP). Our results confirmed that using single polymers alone was unable to achieve satisfactory flocculation. The solids content in the sediment was <30%, even with increasing flocculant dosages up to 2000 ppm for APAM, and 5000–8000 ppm for NHP, respectively. Although better flocculation was obtained by dual polymers, with reduced total flocculant dosages, the improvement was also limited, in terms of the final sediment solids content (<44%). However, much improved flocculation performance was obtained by increasing the SFR value of the MFT (from 0.1 to 1.5), with significantly reduced total polymer dosages at only 650 ppm. Using different combinations of SFR values and dual polymer dosages, the sediment solids could reach 56 wt % after 24 h of natural settling. It was found that the dual polymer system was less sensitive to the changes in polymer dosages, and that no “overdosing” phenomenon was observed for the tested dual polymers. To rationalize the synergy from dual polymers and SFR, we propose that loose and extended flocs formed by the large anionic polymer and suspended fines were consolidated by the small and cationic polymer through strong electrostatic attraction with fines and the large anionic polymer. Coarse sands were “cores” to carry and condense the loosely structured flocs of fine solids, and they exclude the entrapped water inside the flocs. Our work demonstrates that combining dual polymers and suitable SFRs may be an effective strategy to produce sediment with high solids contents, reduce the required flocculant dose, eliminate overdosing issues, and achieve fast settling rates.