Atmospheric drying technologies as practiced by Shell and Suncor are promising options to manage oil sands fine tailings. While single-layer tailings deposits have been shown to dry and gain strength effectively, there are questions as to how the strength and water content of the dried layers will change when successive fresh wet tailings slurries are deposited.
A bench-scale testing program was conducted at Shell Canada Energy’s Muskeg River Mine in northern Alberta to study multi-lift drying of thickened paraffinic froth treatment or tailings solvent recovery unit (TSRU) tailings. The tests were conducted under controlled conditions in boxes with varied drainage conditions to evaluate changes in geotechnical parameters as additional lifts were placed and dried. The tailings were placed in lifts of 50 cm. Instruments were deployed in each lift to monitor soil matric suction, water content and pore-water pressure. Additional analysis included vane shear tests and hand-held tensiometer measurements.
This study found that after placement of a second lift, the first lift lost little strength, and quickly recovered to exceed the prior strength. Soil matric suction returned to its prior values, as did water content, although the drier upper portion of a lift may not return to its prior minimum water content. Second and subsequent lifts gained matric suction and strength after placement more quickly than had the first lift. These findings apply to thickened TSRU tailings, and may have been strongly influenced by partial decant water removal prior to lift placement.