ARTICLE pubs.acs.org/EF
Sample Preparation Method for Characterization of Fine Solids in Athabasca Oil Sands by Electron Microscopy Roham Eslahpazir,† Martin Kupsta,‡ Qi Liu,† and Douglas G. Ivey*,† † ‡
Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2V4, Canada National Research Council, National Institute for Nanotechnology, Edmonton, Alberta T6G 2M9, Canada ABSTRACT: This study introduces a new sample preparation method for characterization of fine solids in Athabasca oil sands using electron microscopy. The method uses a combination of microtome and focused ion beam (FIB) techniques to produce samples for both scanning and transmission electron microscopy (SEM/TEM). In this procedure, SEM images and X-ray maps provide general microstructure and compositional information, with detailed analysis provided through TEM analysis of site-specific electron transparent sections prepared using FIB methods. The method is particularly useful for identifying low concentrations of metals and their compounds and is demonstrated for the identification of fine iron oxide particles in the froth stream of an Athabasca oil sands sample after hot-water extraction. Fine particles (45 μm) of the primary froth stream of an oil sands sample by mapping specific elements.11 SEM samples were prepared by mounting oil sands solids in an electron probe mount followed by polishing and then carbon coating before SEM analysis. SEM, coupled with EDX Received: July 7, 2011 Revised: October 11, 2011 Published: October 12, 2011 5158
dx.doi.org/10.1021/ef200994a | Energy Fuels 2011, 25, 5158–5164
Energy & Fuels spectroscopy, is a powerful characterization device for elemental analysis and imaging, but its relatively poor spatial resolution makes it unsuitable for studying particles a few nanometers in size. The lateral resolution for elemental analysis by EDX spectroscopy is limited by the electron beam specimen interaction volume and is not better than hundreds of nanometers in conventional SEM. Another characterization technique widely used for oil sands mineral identification is X-ray diffraction (XRD).11 14 XRD provides quantitative and qualitative information about the structure and relative amounts of crystalline phases. The size of crystalline particles can also be calculated from XRD patterns, by quantifying the amount of peak broadening. The drawbacks in using XRD are that it is difficult to extract information about particles a few nanometers in size, because of peak broadening effects, and only limited information can be obtained about amorphous materials. The detection limit of XRD analysis limits the information attainable from phases with very low concentrations, and because no image is produced with XRD, it is not possible to see the distribution of different phases in the sample. Transmission electron microscopy (TEM) is a versatile characterization technique, particularly when dealing with materials with nanoscale dimensions. Samples can be imaged with subnanometer resolution, and compositional and crystallographic information can be obtained with spatial resolution better than 10 nm. Compositional analysis is performed through EDX analysis and/or electron energy loss spectroscopy (EELS), which has the added benefit of providing information about the electronic structure of the elements. Crystallographic information is collected from electron diffraction patterns, either selected area or convergent beam patterns. The main limitations of TEM are related to specimen preparation (electron transparent sections of the region of interest are needed) and the small specimen volumes examined, which brings into question the representative nature of the samples. From the early 1990s, TEM has been used by some researchers to study fine solids in oil sands samples.7,15 19 The association of fine solids with the extracted bitumen phase was studied. Kotlyar et al. used TEM for imaging of fine particles and EDX spectroscopy for elemental analysis, but the sample preparation method was not provided in the papers.15,16 On the basis of their EDX results, fine particles in a coker feed bitumen sample were identified as aluminosilicate, pyrite, and titanium oxide.16 Fine solids in the bitumen phase were reported to be covered partially by humic/asphaltene patches, which make them biwettable by water and the organic phase.17,18 Electron microscopy results categorized these solids into four major parts: ultrafine clays, fossils, heavy minerals, and aggregates.18 Pyrites and titanium oxides were the two common minerals identified in the heavy mineral fraction, while aggregates were proposed to contain quartz, clay, and heavy minerals bound together by calcium carbonate, siderite, iron oxides, and humic materials.18 In addition to bright field (BF) imaging and EDX spectroscopy, electron diffraction patterns were used to identify aluminosilicate clays and titanium oxides.17,18 On the basis of the results presented, it is likely that TEM samples were prepared by dispersion of the fine particles onto carbon-coated copper grids. In recent studies conducted by Ivey et al., TEM was used to characterize minerals in the fine fraction (
