Résumé - Abstract
Fluid fine tailings (FFT) generated during bitumen extraction are eventually incorporated into aquatic and terrestrial reclamation landforms. The FFT contains inorganic solids, oil sands process-affected water (OSPW) and residual bitumen. The solids contents, which range from 25% to 35 % (w/w), are dominated by clay minerals (i.e., kaolinite, illite, and illite-smectite mixed layer) and quartz with minor carbonates, trace sulfides and oxides. Associated OSPW contains elevated concentrations of Na+, Cl-, HCO3-, and NH4+, plus trace elements, naphthenic acid (NAs), and residual hydrocarbons. The elevated concentrations of Na+ relative to other cations hinder clay minerals from aggregating. The resulting slow settlement and dewatering behaviour impede progressive reclamation. Consequently, FFT inventories in tailings ponds roughly doubled to 1.27 billion m3 from 2009 to 2019. Regulators have issued directives intended to curtail these growing inventories and promote progressive mine reclamation. These directives, coupled with slow settlement behaviour, prompted oil sands operators to develop various dewatering technologies to manage fluid tailings inventories in active tailings ponds and promote progressive reclamation. These technologies often utilize chemical coagulants and flocculants, yet their impacts on water chemistry and gas production are poorly understood. This research uses batch experiments to examine how different coagulants (i.e., alum [Al2(SO4)3.nH2O], ferric sulfate [Fe2(SO4)3.nH2O], and gypsum [CaSO4.2H2O]) and doses (i.e., 0, 500, 1000, and 1500 ppm) affect FFT geochemistry, mineralogy and microbiology over time. We found that protons contributed by initial acidic solutions and hydrolysis of Al3+ and Fe3+ promote carbonate-mineral dissolution and associated increases in dissolved Ca2+, Mg2+ and Fe2+ concentrations. For the 1000 and 1500 ppm alum batches, the addition and generation of protons consume the acid neutralization potential of the carbonates, and pH consistently decreases below 6. For all other batches, pH remains above 6 and sulfate reduction and sulfide-mineral precipitation influence water chemistry. Sulfate reduction inhibited methanogenesis in batches where SO4 concentrations remained above ~60 mg L−1. Additionally, H2S concentrations decrease in all alum and ferric sulfate batches with elevated Fe2+ concentrations and increase with gypsum batches where Fe2+ concentrations are limited. These results improve our understanding of the biogeochemical implications of chemical coagulants used by the oil sands industry and offer opportunities for improving geochemical outcomes of FFT dewatering technologies.