The one characteristic of oil sand fine tailings which most determines its undesirable properties is its stability. Oil sand tailings sludge is persistent. A major objective of fundamental research on fine tailings is therefore to discover which components and which interactions between components are responsible for this stability. It was shown previously (1) that sludge settling over a two year period could be described by an equation derived by considering sludge to be an aggregation of aggregates of fine particles. These aggregates are postulated to rearrange slowly From a fluffy to a more dense structure. Implicit in this model is that the fine particles are not independently dispersed but rather they are, however weakly, bound to each other. This contribution describes the progress that has been made in understanding these aggregates. Potential structures are illustrated and a mechanism is described by which the aggregates may acquire their strength. It discusses the need for the presence of attractive Forces and the candidate components and mechanisms for providing this attraction. Finally the role of colloid size particles in stabilising sludge structure will be discussed.

Structure of Aggregates

The most abundant clay mineral in sludge is kaolinite, a disc shaped mineral for which the diameter to thickness ratio is about 6 to 8 (2). We will consider these particles as the basic building block of the aggregates.

There are three geometric ways in which to connect disc-shaped particles. They are: lace to face, edge to face, and edge to edge. Face to face stacking of kaolinite leads to a dense deposit. It is the form in which kaolinite is found in the well known Georgia clay and in the McMurray formation. One could say that the most desirable outcome of sludge treatment would be that the kaolinite has formed "booklets" of lace to face oriented particles. These booklets are not, however, what causes sludge and they will not be considered further.

Edge to face geometry is of considerable interest for the study of low density deposits because a small number of particles can occupy a large volume. It has been dubbed "cardhouse" structure and is fundamental to understanding the strength of sludge. Edge to edge geometry leads to chainlike structures. When chains become intertwined rather than aligned they too can occupy considerable volume for their mass. However the strength of their structures is not expected to be greatly affected by the chemistry of the water phase. A factor against the exclusive occurrence of chains is that there is so much more surface area on the face of the disc than there is on the edge that, all other things being equal, the probability of edge to edge connection is only a fraction of edge to face connections. It has been estimated that chains seldom grow to more than 6 links (3) A sketch of aggregates showing all three geometries is given in figure 1 (from (4)). There is at this moment no good reason to rule out the presence of anyone structure in sludge.

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