Water availability and water storage will have impacts on oil sands development as surface-mined production increases. As a result, several technologies designed to increase the percentage of recycle water are being evaluated. One such option is the use of centrifuges to produce dry stackable tailings. Dry stackable tailings can accommodate overburden and soil replacement, and ultimately lead to the restoration of the original boreal forest. Laboratory investigations and lab scale pilot studies have demonstrated that dewatering of mature fine tailings by centrifugation is a function of both the clay content and chemical properties of the tailings being dewatered. The use of additives to improve centrifuge performance has significantly improved the results that can be achieved. Laboratory-scale successes were confirmed with field scale pilot testing recently completed by Syncrude. This paper discusses the laboratory scale centrifuge performance, along with the field pilot results.
Tailings management in surface-mined oil sands is complicated by mine site topography, fines and clay content in the ore, and ore water chemistry(1). The latest ERCB (Alberta Energy Resources Conservation Board) tailings directive proposes to monitor tailings management in much the same way as bitumen production is managed(2). In any case, there will be several management strategies that may be implemented to meet this directive, including such things as composite tailings (consolidated tailings, non-segregating tailings, etc), sand capping, accelerated dewatering, mature fine tailings (MFT) drying, and centrifuged MFT(3). This paper discusses the preliminary results of a recent pilot scale verification of the centrifuged MFT technology.
Centrifugation of MFT as a concept was tested in 1991 in order to provide an upper limit to fluid fine tailings handling costs. Although successful it was felt that the capital and operating costs would be prohibitive(4). Centrifuged fluid tailings was tested much more extensively in 1998 to 2001, and was piloted on a 20 ton per hour extraction plant in Vernal, Utah, for the Asphalt Ridge tar sands(5,6). Although there are slight differences in the clay content between Asphalt Ridge and the Athabasca oil sands, it was felt that similar technology would be applicable to the Alberta oil sands industry. Laboratory scale testing confirmed the positive results from Utah, and larger scale testing was quickly commissioned by both Syncrude and Suncor to verify the small scale results(7). With confirmation of the lab or bench scale centrifuge test program, Syncrude initiated a large scale pilot test at approximately 70m3 per hour of MFT (mature fine tailings). This would be equivalent to approximately 700 tonnes of ore per hour, a scale significantly greater than the pilot studies undertaken in Utah.
Surface-mined oil sands convention defines sand as the mineral fraction larger than 44 microns and the fines as the mineral fraction smaller than 44 microns(8). The coarse tailings are predominantly made up of the sand fraction, and the fine tailings are predominantly made up of fines. It has been demonstrated that it is the clay fraction, as a size and mineral that defines the tailings properties, and in particular, the tailings volume and water holding capacity(9).
