Dewatering fine tails by evaporation: A mathematical modelling approach

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  • Oil sand processing in northern Alberta generates a large amount of water saturated fine-grained material (or fine tails) as a byproduct. Fine tails are a mixture of fine mineral particles (<44µm in diameter), small amounts of bitumen, and a large amount of water. Some consolidation of the solids in the fine tails occurs quickly. The initial solids content of fine tails stream entering the tailing pond is approximately 5 % on a gravimetric basis. After one or two years, the solids consolidate to a solids content of 20% to 30%. However, under natural conditions, further consolidation is extremely slow. More than 500 million cubic meters of fine tails at 30% solids content have been produced over last 20 years by two operating extraction plants owned by Syncrude Canada Ltd. and Suncor Inc., in Fort McMurray, Alberta. Furthermore, fine tails are accumulating at a rate of approximately 20 million cubic meters per year. The development of a system to reduce the enormous volume of fine tails, or to dry to a stable surface which permits further remediation treatments, presents a unique challenge for the sustainable management of the environment. Natural evaporation is one of the available options that can be used to reduce the volume of fine tails because it is both inexpensive and technically feasible on a large scale. Evaporation of water from fine tails involves the complex interactions between the potential atmospheric evaporation demand and the physiochemical characteristics of the fine tails. When the surface of fine tails is wet (saturated with water) evaporation is controlled by the energy balance and the rate of vapour transport at the boundary between the atmosphere and the surface of the tails. The calculations needed to quantify dewatering at this stage are uncomplicated and well known. Eventually, evaporation at the surface exceeds the rate of transport of water within the layer of fine tails, and a surface crust develops. At this point further evaporation is controlled by the physical properties of the fine tails which govern the movement of water to the, surface in either a liquid or vapour phase. The formation of a crust and the movement of water under unsaturated conditions are complex and not well understood. A thorough understanding of the complex interactions among the solids, water and external atmospheric demand is required to develop a system to dewater fine tails by evaporation. However, the effects of ponding layers and initial solids content as well as the effect of atmospheric conditions on the desiccation rate of fine tails are not fully understood. Furthermore, experiments can not be conducted for all possible conditions regarding different solids content, ponding depth, and evaporation demands. Thus, a mathematical model is an essential tool to identify potentially important parameters needed to understand these complex processes and identify the conditions at which optimum evaporation may be achieved. A one dimensional approach was used to model the evaporation process from fine tails.

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