Manipulating chemicals dosing sequence for improved aggregation and filtration of oil sands tailings

• Anionic flocculant − cationic coagulant sequence led to much faster filtration rate. • Coagulant adsorbed more strongly when added after anionic flocculant. • Coagulant neutralized the flocculant’s negative charges, enhancing chain rigidity. • The resulting flocs were more shear resistant and maintained a high porosity. Dry stackable tailings is highly desirable as it allows for fast water recycle and facilitates land reclamation. To generate stackable tailings, it is critical to develop rapid and cost-effective dewatering processes. In this study, we investigated the aggregation and filtration of oil sands fluid fine tailings (FFT), a complex waste slurry of extreme colloidal stability, following individual and combined chemical treatment with coagulants and flocculants. We observed that the filtration efficiency depended on the sequence of coagulant and flocculant dosing. When an anionic flocculant (Kemira polyacrylamide) was added before a coagulant (alum, ferric chloride, or polyDADMAC) (F-C sequence), much faster filtration rates and higher solids content in filter cakes were observed than that resulting from the reverse coagulant-flocculant sequence (C-F). Adsorption analysis using QCM-D revealed that the coagulant adsorbed at higher densities and with faster kinetics following the prior adsorption of the anionic flocculant in the F-C sequence. The coagulant neutralizes the negative charges of the flocculant and enhances the rigidity of the integrated adsorption layer. The closer packing and cross-linking of the flocculant chains induced by the coagulant generates a robust floc matrix with higher shear resistance, and maintains the porous structures needed for fast filtration. In contrast, the C-F dosing sequence produces a softer, more flexible adsorption layer where the anionic flocculant readily undergoes conformational rearrangement, leading to a weaker and more deformable floc matrix that is incompatible with fast and efficient filtration. These findings elucidate the relationship between microscopic adsorption layer configuration, floc structure, and macroscopic filtration performance in individual or sequential dual chemical treatment of oil sands FFT at different dosing sequences. The understanding can help optimize chemical conditioning schemes for fast tailings filtration and water recycle, ultimately leading to timely reclamation and sustainable mining practice.