Recovery of Water And Bitumen From the Athabasca Oil Sand Tailings Ponds(a)

Abstract

Introduction Various methods to recover bitumen and water from the oil sand tailings pond were explored. It was shown that removal of free bitumen from the tailings tends to destabilize heavy sludges and also improves drying rates dramatically. Modification of the sludge pH using NaOH showed that improved settling took place only above pH-11. A heavy sludge was fractionated to yield a number of distinct mineral fines fractions, including a solid associated with the oil phase, a fraction which remained in colloidal suspension in the aqueous phase and a fraction which settled from the aqueous phase. All three solids contained significant quantities of organic matter. The oil-phase solid also showed considerable enrichment in heavy metals compared to the other solids. Introduction The hot water process used by Suncor and Syncrude to extract bitumen from the Athabasca oil sands produces large volumes of tailings(1,4). The raw feed consists of bitumen, sand, water and mineral fines. After discharging the tailings, the +44 µ particles settle out readily. When a substantial fraction of particles is present in a finer state of dispersion, aqueous sludges are produced(1) containing mineral fines(3), free and adsorbed bitumen(4), surfactants(5,6), bitumen oxidation products(7) and caustic soda. Although the settling behaviour of aqueous clay suspensions has been studied in detail, for instance, in connection with drilling muds(8), the settling of the tailings-pond sludges is generally not well understood. Recent studies have shown the settling to be non-Stokiam(3), with free and adsorbed bitumen perhaps playing a key role(4). Also, the presence of fine clay platelets, because of their differential edge and face-wetting properties, possibly assists in the stabilization of the suspensionst(3,4). As the recycle water for the extraction process cannot contain more than - 5% solids, a large volume of the tailings must be stored in ponds of ever increasing volume. Some settling occurs to permit re-use of the uppermost pond layer only. With the continuing use of water required for the increased production of bitumen from the Athabasca oil sands, interest in recovering process water is mounting(2,4). From an economic point of view, the recovery of additives, such as caustic soda, and residual bitumen is significant in considering any water recovery process. In this communication, we describe various experiments carried out to study and improve the settling behaviour of the sludges. These experiments include free settling with pH modification evaporation and some simple forms of pretreatment to recover bitumen. In addition, we investigate the nature of the fines present in the tailings-pond sludge. Experimental Aqueous tailings in 5-gallon to barrel-size lots were obtained from the Suncor plant sludge pond, and contained 5–12% solids by weight, although in some lots the solids content ranged as high as 30%. Solids contents were obtained by drying aliquot samples to constant weight at 110°C. The organic content of oven-dry samples was determined using several methods, including toluene extraction, loss on ignition (LOI) at 450°C and carbon determination by combustion followed by gravimetric measurement of evolved CO2.