Ozone-Assisted Settling of Diluted Oil Sands Mature Fine Tailings: A Mechanistic Study

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  • The accumulation of mature fine tailings (MFT) in tailings ponds with very slow natural consolidation rate has been a concern for decades. Previous studies and current practices for tailings consolidation focus on the addition of various inorganic and organic additives to reduce MFT production and accumulation. It is highly desirable to develop more efficient and effective techniques for MFT consolidation to reduce the potential environmental impacts of oil sands tailings. In this study, the performance and mechanisms of a novel ozone-assisted MFT dewatering method were evaluated. MFT with different solids contents were ozonated for 15, 30, and 60 minutes to determine the optimal ozonation treatment conditions for MFT consolidation. The volume and turbidity of release water, and the solids content of settled sludge (i.e., solids) were measured to evaluate the performance of ozone on MFT consolidation. MFT initial settling curves were developed by monitoring the change of the water and sludge interface positions over time. To investigate the surface interaction mechanisms involved in the process, major ions of MFT release water and the surface charge of MFT fine particles were characterized. Specifically, these parameters included pH, ion concentrations, acid extractable fraction (AEF) concentration of MFT release water, zeta potentials, and surface functional groups of fine particles. Additional experiments were performed to investigate the roles of pH and ion concentrations in ozone assisted MFT consolidation. Our results showed that ozonation treatment effectively accelerated diluted MFT particle settling at all treatment conditions tested in this study. MFT quickly settled after short (15 minute) ozonation treatment, leaving clear brown water and compact settled sludge. The volume and turbidity of release water and the solids content of settled sludge were comparable at 1 wt% MFT under different ozonation times (15, 30 and 60 minutes). At 3 wt% and 5 wt% MFT, a longer ozonation time improved MFT settling. Therefore, we conclude that the ozonation time needed for MFT settling varied depending on the solids content of MFT suspensions. Additional studies showed that the ozone-accelerated settling of MFT particles probably results from a change in MFT particle surface properties and the weakening of repulsive forces among fine particles through at least four mechanisms: (1) the desorption of stabilized organic matter on MFT surfaces, which reduces the steric or electrostatic stabilizing effects among fine particles; (2) the release of organic acids (such as oxalic and acetic acids) into the water, which decreases the pH of the MFT suspensions and neutralizes MFT surface charges; (3) ozone breakage of organometallic complexes in MFT, leading to the release of oxidized metal ions such as Fe2+, Mn2+, and Al3+ that can act as coagulants; and, (4) an increase in cations and carboxylic groups that facilitate the generation of metal humate complexes, leading to an accelerated precipitation of fine particles. Compared to the chemical additives currently used or studied to accelerate MFT consolidation, ozone-based processes have several advantages: (1) The process is economical because even a low ozone dose appears to be sufficient to destabilize tailings fine particles; (2) The process is environmentally friendly because ozone decomposes rapidly; and (3) The excess ozone can decompose organic compounds in tailings and thus improve the water quality. Despite the promise observed in the present study, a cost analysis and a life cycle analysis should be performed to determine the feasibility of this treatment process. The effectiveness of ozonation treatment for undiluted MFT settling should be evaluated. The toxicity of the release water should be evaluated to examine the environmental impact of the process. Additional research on the long-term impact of ozone-assisted MFT settling processes on MFT reclamation is also needed.

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    Attribution 3.0 International