Structure And Properties of Oil Sands Clay Tailings
Authors: Kessick Michael A. 
Published: March 2010

Oil sands clay tailings settle to a gel-like sludge or slime which is still relatively low in solids content, and the tailings show extremely slow dewatering beyond this point. These characteristics imply a significant degree of structure in the slime. Based on a comparison of some of the rheological and stability characteristics of the material and those of a bentonite gel over a range of pH, a structure is proposed which depends for its stability on the presence of residual bitumen as well as a clay-bound organic component.

Introduction

The clay slimes associated with the hot water extraction of Athabasca oil sands result from the settling of the clay fraction of the process tailings. They are of gel-like consistency, still containing up to 80% water. Further dewatering beyond this point is extremely slow, and the slimes have already accumulated to a massive degree as a result of the Great Canadian Oil Sands Ltd. (GCOS) operation. The accumulation is described in detail by Camp(1).

The oil sands clay slimes have been likened to similar slimes produced by other extractive industries, including phosphate, diamond, china clay and bauxite refining(2). Although impoundment and disposal techniques for these wastes are in general similar throughout the industries, the compositions of the various slime types are sufficiently different to warrant individual and detailed examination if attempts are to be made to dewater them further or to reduce the volume produced. Dewatering is desirable in that it would release large quantities of water locked up in existing slime ponds. Volume reduction would also reduce the need for the very large and environmentally worrisome impoundment areas now used.

Some of the slimes are readily identifiable as having their properties controlled by the presence of swelling clays. Phosphate slimes owe their characteristics to the presence of attapulgite and montmorillonite(3), both important as swelling clays, and used accordingly in drilling muds as rheology control agents. Saponite, another well-characterized three-layer swelling clay of the smectite group, has been identified as the structure-forming agent in diamond mine slimes(4). On the other hand china-clay slimes owe their properties mainly to the presence of micas and kaolinite(5); -bauxite (red mud) slimes consist predominantly of hematite, complex aluminum silicates, silicate hydroxides and compound carbonates, with some evidence for the presence of kaolinite, chlorite and hydrous micas(6).

In the case of oil-sands slimes, montmorillonite has been implicated as a factor in the poor dewaterability of these materials(l). Its role has remained unclear, however and close inspection of available data would seem to indicate that stable oil-sands slimes may be formed in the absence of any well defined swelling clay. There is evidence that sediment volumes, Technology, January-March, 1979, Montreal and consequently water retention, are related to structure formation (7). The structure in non-Newtonian liquid systems is also reflected in their flow properties. This study therefore compared certain rheological and stability characteristics of oil-sands slime, which contained no detectable swelling clay content, with a relatively pure montmorillonite gel.