Factors affecting Athabasca oil sand sludge stability.

The two major oil sand extraction plants, Syncrude Canada Ltd. and Suncor Inc., utilize the Clark Hot Water Extraction Process to remove bitumen from surface mineable Athabasca oil sands. This process produces a waste stream containing dispersed clay, sand, silt, and bitumen which is pumped to tailings ponds. The coarse sand and some of the finer particles settle out and are used to construct containment dykes around the ponds. However, much of the clay, silt, and bitumen remain in suspension and are carried out towards the centre of the pond. These solids eventually form a sludge with a gel-like structure, retaining from 85 to 90 percent of the recyclable process water. The ever-increasing, enormous tailings ponds present a serious environmental hazard to the nearby lakes, rivers, land, and wildlife. It has been proven that ultrafine (10 to 200 nm) clay particles, capable of forming a highly porous, three-dimensional gel network, are the main components responsible for structure formation within Athabasca oil sand sludge. The ultrafine clay particles, mainly kaolinite and mica, are able to form the gel network in a short period of time (1 to 14 days) and the gel has a high water holding capacity. The oil sand sludge is also composed of coarse solids, poorly-crystalline inorganic components, and strongly-bound organic matter. The deuterium nuclear magnetic resonance technique was used to study the effect of these components, as well as, the effect of the different sizes of ultrafine clay particles on gel formation. It was observed that the gel forming propensity of the sludge was significantly increased with a decrease in particle size of the ultrafine clays. The presence of coarse solids slowed the rate of gel formation; however, the poorly-crystalline inorganic components were shown to have no effect on the gel forming propensity. The ultrafine clay particles were further separated into hydrophilic and biwetted solids. The biwetted ultrafine solids were identified as having surfaces that contained a large percentage of strongly-bound organic matter. The presence of these particles significantly accelerated the rate of gel structure formation. Lastly, water chemistry (chemical composition of dissolved matter) was proven to play a significant role in sludge structure formation. The ultrafine clay particles would only form a gel when the aqueous media contained an appropriate amount of dissolved electrolytes. Gel structure formation was prevented in an aqueous media of distilled water. As well, a significant increase in the electrolyte concentration of the aqueous media induced coagulation, thereby allowing a dense sediment to be formed and a large volume of trapped water to be released. These results have revealed that the combination of ultrafine clay particles and appropriate water chemistry is essential before structure formation can occur within oil sand sludge