Approximately 12 barrels of water are used for the production of each barrel of bitumen in surface mined oil sands operations. Typically, about 70% of this water is recycled, leaving a net trade of about 4 barrels of water per barrel of bitumen production. This water is tied up in the pore spaces of the mineral sand, silt, and clay left after the bitumen is extracted from the oil sands. The sand component of the mineral tailings is a relatively straightforward reclamation problem due to the relative ease with which strength can be developed in the sand tailings. The silt and clay component which forms the fluid fine tailings, or mature fine tailings, is usually contained behind large dikes and at 30 to 50% solids, even after 40 years, this material does not have enough strength to support the overburden or soil horizon replacement required for reclamation. Currently the lowest cost reclamation option is the long term storage of the mature fine tailings under a water cap in an end pit lake. This so-called wet landscape reclamation has several unknown long term monitoring liabilities, aside from the difficulties inherent in the creation of an artificial lake above the mature fine tailings. Some of the tailings management options which would lead to a dry stackable tailings naturally also significantly decrease the barrels of water lost with each barrel of bitumen production. These options are discussed along with an analysis of their impact on recycle water quality and quantity.
Tailings management in surface mined oil sands is a very complex endeavor (1). In order to appreciate the impact of various tailings management options, it is possible to apply several simplifying assumptions. In this discussion, the tailings management options and their impact on recycle water chemistry will necessarily have to be simplified, and where possible, the shortcomings of these simplifications will be noted.
In general, there are three oil sands tailings streams. These are the coarse tailings from the primary bitumen separation step, the fine tailings from the secondary and/or tertiary bitumen recovery step, and the froth treatment tailings. Solvent addition to the bitumen froth reduces the viscosity and allows for the removal of the froth minerals. The resulting froth treatment tailings are particularly important because of the environmental impact of the residual solvent or diluent. In terms of their contribution to the total tailings volume, however, they represent a relatively minor stream(2,3).
Surface mined oil sands convention defines sand as the mineral fraction greater than 44 microns and the fines as the mineral fraction less than 44 microns. 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. For comparison purposes, however, one can discuss the fines fraction, with the underlying assumption that the clay to fines ratio is 0.5.
