Reduction of Environmental Impacts of Oil Sands Plants by Implementing Bitumen Extraction and Tailings Disposal Processes using Lime (CaO) Additive

S. Arnipally, A. Tang, J.D. Scott, B. Ozum

In the proceedings of: GeoEdmonton 2018: 71st Canadian Geotechnical Conference; 13th joint with IAH-CNC

Session: Mining Geotechnics and Hydrogeology II

ABSTRACT: In Alberta, Canada, bitumen is produced by surface mining of Athabasca oil sands ore followed by ore-water slurry-based extraction processes, at a capacity exceeding 1.3x106 barrels/day. Clark Hot Water Extraction (CHWE) process developed in 1930s is used at all oil sands plants, which uses caustic NaOH as an additive to keep the pH of the extraction process slurry at about 8.2 to 9.0. Increase in slurry pH activates naphthenic acids in bitumen, which act as surfactant species reducing water surface and bitumen-water interfacial tensions. Reduction in surface and interfacial tensions promotes clay-water and bitumen-water wettability. As a result, NaOH additive promotes liberation of bitumen from oil sands matrix; however, it promotes silt-clay size particles dispersion and slows down coalescence and aeration of bitumen droplets, which harms overall efficiency of oil sands plants. Also, CHWE process causes accumulation of Na+ in recycle process water, which detrimentally effects bitumen extraction and promotes production of toxic mature fine tailings. Existing mature fine tailings inventory is exceeding 109 m3 and its production rate is exponentially increasing as bitumen production capacity increases. At our laboratory, bitumen extraction process is being studied by using lime (CaO) as additive replacing NaOH of the CHWE process. Use of CaO as an extraction process additive eliminates Na+ accumulation in recycle water, reduces surfactant activities of water soluble naphthenic acids, suppresses silt-clay size particles dispersion in the extraction process slurry, reduces mature fine tailings production, promotes bitumen extraction efficiency and increases the kinetics of bitumen droplets coalescence and aeration processes. Furthermore, tailings disposal processes, such as production of nonsegregating tailings, whole tailings treatment, and mature fine tailings dewatering processes were studied, also using CaO additive at different dosages. Experimental data will be presented by focusing on practicality, integration to the existing plants, capital and operating cost reduction perspectives. Key Words: oil sands, bitumen extraction, oil sands tailings, mature fine tailings, process affected water 1 BACKGROUND The Northern Alberta, Canada oil sands resources which extend over 77,000 km2, distributed in three principle regions: Athabasca, Cold Lake and Peace River. The estimated crude bitumen in-place volume and ultimate potentials for recovery are 270x109 m3 (1.6x1012 bbl) and 50x109 m3 (300x109 bbl), respectively (Precht and Rokosh, 1998). The Athabasca, McMurray deposits are surface mineable, where the overburden thickness is less than 75 m with in-place bitumen volume of about 24x109 m3 (144x109 bbl). In deeper formations, where the crude bitumen is not mobile in the reservoir, thermal energy application is required for the recovery of the bitumen. In Alberta, bitumen is produced by surface mining followed by ore-water slurry-based extraction processes at over 1.3x106 bbl/d capacity. Production capacity is predicted to grow as the existing plant capacities are increased and new plants become operational. Some versions of Clark Hot Water Extraction (CHWE) process of 1930s are used at all oil sands plants, which uses caustic NaOH as extraction process additive (Clark, 1939; Clark and Pasternack, 1932). All bitumen production plants are permitted to operate with zero discharge policy, by which the process water recovered from oil sands tailings are recycled. This operating practice causes accumulation of Na+ concentration in process affected water. Use of NaOH as an extraction process additive and increase in Na+ concentration in process water cause problems for both bitumen extraction and tailings disposal processes. CHWE process disperses silt-clay size particles in extraction process slurry, which promotes production of toxic mature fluid fine tailings (FFT). Mature FFT is composed of about 33% solids (84% water by volume), over 98% of which are finer than < 45 µm size particles (-325 mesh) and dewaters with very slow rates; because of which existing mature FFT inventory reached over 109 m3 over the five decades. Existing toxic mature FFT stands as a serious environmental liability for both oil industry and all level of governments. In summary, use of CHWE process, more specifically use of NaOH as an extraction process additive, is the source of environmental and long terms sustainability problems associated with the operations of oil sands plants. Two major attempts were made by the oil industry to overcome these problems: (i) development of non-additive extraction processes to reduce or supress dispersion of silt-clay size (< 45 µm) particles in the extraction process slurry, by which extraction plant produces tailings material with improved geotechnical characteristics, reduces MFT

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