Abstract

Freeze fracture preparation and subsequent direct observation of frozen hydrated samples in an electron microscope has been used to characterize the morphology and composition of oil sands sludges. A card house type of structure has long been suggested to explain the long term stability of sludge to consolidation but the relative role of organic/mineral or mineral/mineral interactions in determining the stability of the structure is not well understood.

Electron microscopic techniques are discussed which might allow direct determination of the relationship between the various sludge components_ Evidence is presented which suggests that it is possible to directly observe the sludge structure.

Introduction

The sludge component of oil sands tailings is only a small part of the various process streams associated with production of synthetic crude oil from oil sands (1–3). However, it is a particularly stable component and since it is very slow to consolidate, it has continued to accumulate. Given the huge volumes of oil sands processed every day, even a small sludge fraction will eventually build to a significant volume. An understanding of the physical properties which give rise to this stability is important in elucidating the relative importance of the role of organic material in determining sludge properties versus simple mineral characteristics. It is well known that kaolinite clay platelets can align positive edges with negative faces to build a so called: card house" structure. This idealized structure is unlikely in sludge due to the complexity of the system; the modification of the ideal card house suggested by Scott et a1 (1) is more likely to exist in sludge. By using some specialized microscopic techniques it is hoped that the sludge structure might be directly observed and ultimately characterized to establish the fundamental reasons for the structure formation. This understanding might lead to resolution of the sludge build up by indicating a method for destabilizing the sludge structure. This study provides some tantalizing direct evidence for sludge structure which warrants further investigation.

Preliminary results suggest a model of sludge structure involving a hierarchy of structures based loosely on the card house type of unit but with somewhat longer range ordering of the card house "building blocks". This longer range ordering can be relatively easily disturbed and broken up, yet it rapidly re-forms.

The rheological behaviour of sludge in terms of increased yield stress and thixotropy after time is indirect evidence of some sludge structuring (4). This behaviour can now be thought of in terms of rearrangement of the sludge "building blocks" into longer and longer range structures as a function of time.

Results are discussed on the direct observation of these structures in oil sands sludges and related model systems which indicate that the card house type of structure does exist and can be characterized using microscopy.

Potential artifacts introduced by the freeze fracture sample preparation technique and their effect on the interpretation discussed.

Experimental Methods

In this study, the approach was to use direct observation of frozen hydrated specimens in a scanning electron microscope.

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