Article pubs.acs.org/EF
Characterization of Iron-Bearing Particles in Athabasca Oil Sands Roham Eslahpazir, Qi Liu, and Douglas G. Ivey* Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2-V4 ABSTRACT: Iron-bearing particles in two Athabasca oil sands ore samples have been characterized in this study by electron microscopy. One sample was taken from a “good processing ore” and one from a “poor processing ore”. These samples were subjected to a batch extraction process and the resulting solids in the primary froth were characterized. While iron-bearing minerals such as magnetite and hematite were common in both ore samples, pyrite and goethite were only found in the poor processing ore and wustite was identified only in the good processing ore. The iron-bearing particles were concentrated in the primary froth stream from batch extraction, and their sizes varied from a few nanometers to several hundred nanometers. The nanometer scale iron bearing minerals have been identified in two different arrangements. Nanoscale iron-bearing minerals either form patches on top of relatively large (200−300 nm) clay particles, or they combine with nanoscale clay and toluene insoluble organic material to form mineral-organic aggregates.
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INTRODUCTION Scientific research and commercial investment in the Athabasca oil sands have a history of more than one hundred years,1 and yet, the oil sands industry is still developing new processes to increase efficiency and to minimize environmental impact. Research focused on characterization of oil sands ores provides a better understanding of oil sands components and has been playing an important role in new developments. Mineral solids constitute up to 70% by weight of the oil sands2 and have been characterized extensively since the early years of bitumen extraction from oil sands. Fine mineral particles, less than 2 μm in size, have been the subject of detailed studies because of their large specific surface areas and their tendency to associate with the organic phase. Clays are the most abundant particles in oil sands fines, and their impact on bitumen liberation in the presence of divalent metal ions3−5 and the roles they play in tailings dewatering have been studied extensively.6,7 Clays are accompanied by other fine particles, such as nanoscale iron-bearing particles and titanium oxide minerals, in the fine fraction of oil sands solids. Fine iron-bearing minerals have been identified by X-ray diffraction (XRD) in oil sands samples, but they have not been studied as in-depth as clays.8,9 Ferric ions are not stable in solution and form ferric hydroxides at pH values around 3.10 Iron hydroxides precipitates are suggested to play the same role as divalent metal ions in bridging bitumen and solids.11,12 It has been demonstrated that the ζ potentials of bitumen droplets in a suspension rich in iron ions followed the same trend as iron hydroxide particles, which supports the observation that iron hydroxides cover the surfaces of bitumen droplets.12 Characterization methods such as scanning electron microscopy (SEM) and XRD have been used to identify clays, and their relation with the organic phase in the fine fraction of oil sands streams.13−15 One problem with SEM is its relatively poor spatial resolution, which makes it impossible to obtain composition information from particles that are a few nanometers in size. The spatial resolution for energy dispersive X-ray (EDX) analysis in the SEM is typically about 1 μm for a © 2012 American Chemical Society
20 kV electron beam. XRD has its own limitations when characterizing materials such as nanoscale iron oxide particles. These include no imaging capability, difficulty in characterizing amorphous or nanocrystalline materials, and inadequate detection limit for phases present in low concentrations. Transmission electron microscopy (TEM) is a versatile technique when dealing with characterization of nanoscale ironbearing particles in oil sands samples. TEM is capable of detecting phases present in low concentrations, and it has subnanometer spatial resolution for imaging, crystal structure analysis, and composition analysis. Bright field (BF) and dark field (DF) imaging can be used to show the morphology, size distribution, and association between different particles. Electron diffraction, either selected area electron diffraction (SAED) or convergent beam electron diffraction (CBED), can be used to collect crystallographic information. Compositional analysis is done using EDX and/or electron energy loss spectroscopy (EELS), which has the added benefit of providing information about the electronic structure of the elements. EDX analysis in the TEM has much better spatial resolution (45 μm) and smaller than 45 μm (
