The dewatering of oil sands sludge is a major technological, economical, and environmental challenge to the oil sands industry of northeastern Alberta. Sludge is a mixture of small mineral particles (less than 44 µmin diameter), residual bitumen from the extraction process, and water. Sludge consolidates at the bottom of tailings ponds to approximately 30% solids in 2 years and will remain at this level of solids and water indefinitely. At 30% solids, sludge acts as a liquid; unstable and extremely low in strength. Approximately 25 million cubic metres of sludge at 30% solids are produced each year by the two operating extraction plants owned by Syncrude Canada Ltd. (SCL) and Suncor Inc. More than 500 million cubic metres of sludge have been produced over the first 20 years these plants have operated.
The experiments detailed in this report show that it was possible to increase the solids content of sludge to 50% solids by adding three parts sand (tailings sand) to one part sludge. At 50% solids, the sand-sludge mixture was semi-plastic, but extremely weak. One thousand parts per million of lime were needed to keep the sand from segregating from the sludge. Drainage of sand-sludge mixtures, even under the pressure of self-consolidation, was slow and uneconomical. The sand-sludge mixture had to be dewatered to 85% solids content before its shear strength was sufficiently high to support machine traffic or the overboarding of more sand-sludge mixture. At 85% solids, the sand-sludge mixture had a shear strength in excess of 100 kPa.