Abstract
Oil sands surface mining operations in northeastern, Alberta, Canada produce enormous volumes of fluid fine tailings, an aqueous suspension of fine clays, sand, unrecovered bitumen, and diluent hydrocarbons. The tailings are deposited and retained on-site in large settling basins where the colloidal solids sediment and consolidate very slowly by gravity and pore water collects at the surface for re-use. Tailings ‘biodensification’, mediated by indigenous microbes that produce methane and/or carbon dioxide, is a phenomenon observed in situ and in vitro whereby tailings with active anaerobic microbial communities consolidate and de-water faster than predicted by gravitational (self-weighted) consolidation alone. To exploit this phenomenon, we used organic amendments to stimulate endogenous anaerobic tailings microorganisms. Tailings from three different operators were amended with agri-business by-products, placed in 100-mL microcosms and 1.5-L settling columns, and monitored for methanogenesis, pore water recovery, and solids densification. Several amendments increased methane production and accelerated biodensification compared to unamended and negative controls. Hydrolyzed canola, blood meal, bone meal and glycerol generally accelerated biodensification, stimulated methane production and supported growth of methanogens and fermentative microbes. Amendment altered the chemistry of the tailings, generally decreasing pH, increasing conductivity and magnesium, potassium, sodium, and bicarbonate concentrations. Biodensification is a potential engineered technology for accelerating water recovery and reducing the volume of stored oil sands tailings.
Highlights
The Athabasca oil sands deposits in northeastern Alberta, Canada comprise a vast reserve of biodegraded petroleum, known as bitumen, that is surface-mined by several companies
Fluid fine tailings comprising suspended sand, solids, clays, unrecovered bitumen, and residual diluent are stored in large settling basins holding hundreds of millions of cubic meters of tailings
Sources of oil sands tailings and organic amendments Tailings ponds operated by three different oil sands companies (Syncrude Canada Ltd., Suncor Energy Inc., and Shell Albian Sands) were sampled for mature fine tailings below the water:sediment interface (“mud line”) and process-affected water at the surface, using sterile containers filled to overflowing to eliminate aerobic headspace, per Foght et al, (2014)
Summary
The Athabasca oil sands deposits (colloquially called “tar sands”) in northeastern Alberta, Canada comprise a vast reserve of biodegraded petroleum, known as bitumen, that is surface-mined by several companies (reviewed by Foght et al, 2017). Bitumen extraction requires huge volumes of water, including recycled and makeup water from the Athabasca River to separate bitumen from the sand, clay and minerals of the ore. A light hydrocarbon diluent, the composition of which differs between mining companies, is used in one processing step to reduce bitumen viscosity and enhance separation of bitumen from fine solids. Fluid fine tailings comprising suspended sand, solids, clays, unrecovered bitumen, and residual diluent are stored in large settling basins (tailings ponds) holding hundreds of millions of cubic meters of tailings. Thereafter, the solids (predominantly fine clay particles) suspended beneath the surface water layer form a semi-solid colloid that consolidates very slowly while expressing pore water; gravitational densification of fines to a “trafficable” physical state has been estimated to take 125-150 years (Eckert et al, 1996).
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