5 Instrumental analytical techniques
5.2 Electron microscopy
An electron microscope is a type of microscope that uses an electron beam to illuminate a specimen and produce a magnified image. There are several types of electron microscope such as transmission electron microscope (TEM), scanning electron microscope (SEM), and reflection electron microscope (REM), with SEM being most extensively used in coal analysis.
5.2.1 Scanning electron microscopy
The SEM produces images by probing the specimen with a focussed electron beam that is scanned across a rectangular area of the specimen (raster scanning). When the electron beam interacts with the specimen, it loses energy by various mechanisms. The lost energy is converted into alternative forms such as emission of low-‐energy secondary electrons and high-‐energy backscattered electrons, light emission or x-‐ray emission, which provide signals carrying information about the properties of the specimen surface, such as its topography and composition. SEM has been extensively used to study the morphology and cross section of coal and coal char/ash, and analysis of the SEM image can provide valuable information such as particle size, swelling of the particles during conversion, and the structure of char/ash. This information can be correlated to the fundamental properties of parent coal and the process parameters.
SEM with energy dispersive x-‐ray spectrometer (EDS) has been widely used to identify the composition and nature of minerals in coals and to determine the associations of the minerals with each other.
Examinations can be made of samples resulting from ashing techniques or whole coal. With this technique it is possible to identify the elemental components and deduce the mineral types present in coal samples. Figure 18 shows an example of the SEM image of a cross-‐section of a bituminous coal and its x-‐ray spectrum obtained using an SEM-‐EDS. Recent studies on iron sulphide minerals in coals, minerals in coals, and transformation of minerals during coal conversion all used SEM as the primary analytical tool (Kgabi and others, 2009; Zhu and others, 2002; Manoj, 2013; Manoj and others, 2009; Oliveira and others, 2011; Ma and others, 2013).
Figure 18 – SEM-‐EDS characterisation of an Indian bituminous coal (Manoj, 2013)
Advantages of a SEM include its wide-‐array of applications, the detailed three-‐dimensional and topographical imaging and the versatile information garnered from different detectors. SEMs are easy to operate, work fast, and require minimal sample preparation. In addition, the technological advances in modern SEMs allow for the generation of data in digital form. However, SEMs are expensive, large and must be housed in an area free of any possible electric, magnetic or vibration interference. Special training is required to operate an SEM as well as to prepare samples as the preparation of samples can result in artifacts. Also, SEMs carry a small risk of radiation exposure associated with the electrons that scatter from beneath the sample surface.
Recently, computer-‐controlled SEM (CCSEM) to evaluate SEM images have been developed to determine the size, quantity, distribution, and semi-‐quantitative composition and association of mineral grains in coal mine dusts, coals, coal ashes and in coal liquefaction residues. With this information, the impacts of coal properties on wear of system components, slag flow, fouling of heat exchangers, fine-‐particle collection, and ash handling can be assessed and predicted.
A CCSEM instrument consists of an SEM that is interfaced with a computer-‐controlled acquisition and data-‐collection system. The system can measure several thousand different mineral particles, which appear randomly placed in a polished section of the coal embedded in resin or wax. In the SEM, the electron beam is under the control of a computer and is stepped across the field-‐of-‐view in a relatively
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coarse grid in order to locate mineral particles in the coal/resin (or wax) matrix. The back-‐scattered electron intensity is used to discriminate between mineral particles and the background of coal macerals and resin or wax mounting medium. Once a mineral particle is located, the grid spacing is greatly reduced.
The cross-‐sectional area of the particle is measured by extending eight diagonals through the centre of the particle to the edge of the particle and summing the areas of the triangles so produced. After the area is measured in this manner, the electron beam is positioned at the centre of the particle and an EDS spectrum is obtained. The chemical information contained in this spectrum is then used to identify the mineral. By analysing several thousand particles in the coal section in this manner, a reasonably quantitative description of the coal mineralogy can be obtained (Gupta, 2007). A diagram of a CCSEM system is shown in Figure 19.
Figure 19 – A diagram of a CCSEM system (MTI, 2012)
An international co-‐operative study on the accuracy of a CCSEM system for analysis of minerals in coal found that the main factors affecting the accuracy of CCSEM analysis were the coal sample preparation technique and the number of mineral grains analysed. The differences of the CCSEM systems including their operating conditions only played a minor role to account for the discrepancy of CCSEM measurements, as different laboratories gave similar CCSEM results on the same coal sample stub. The results from this study indicated that the sample preparation technique that mixes crushed coal and wax into a pressed pellet produced an optimum particle distribution. If a coal sample contained coarse minerals, then a greater number of grains comprising large size minerals would need to be analysed (Gupta and others, 1999). Improved procedures for sample preparation and CCSEM analysis were developed and described by Cieplik and others (2003) in which Carnauba wax was used as embedding material for increased contrast between embedding material and the organics in coal to distinguish between free mineral particles and those included in the organic coal matrix. Optimised particle distribution and mass fraction were also obtained using the procedures. Since CCSEM was developed around 20 years ago, it has been refined, enhanced, and continuously advanced. One of the improvements has been to move from single elemental analysis at a central point to an average elemental analysis
SEM sample preparation
grinding
mounting wax or resin
polishing 1–µm grit
imaging backscattered electron imaging
particle sizing particle recognition and characterisation chemical analysis
x–ray spectroscopy
raw data size, shape, composition
phase analysis
output
computer models predictive indices, cohesivity,resistivity,
particle size
report
derived from several analyses obtained at a number of points on a grid imposed on each particle. This accounted for variation in the mineral composition within a mineral grain. The mineral recognition programme and general data handling have also been extensively developed over recent years (Gupta, 2007). Recent investigations have shown CCSEM to be a valuable means of identifying phases of minerals in coal and coal ashes (Soundarrjan and others, 2013; Cprek and others, 2007).
5.2.2 Electron probe microanalysis
An electron microprobe (EMP), also known as an electron probe microanalyser (EPMA) or electron micro probe analyser (EMPA), is an analytical tool used to non-‐destructively determine the chemical composition of small volumes of solid materials. It is essentially a hybrid instrument combining the capabilities of both the SEM and an XRF, with the added features of fine-‐spot focussing (approximately 1 micrometer), optical microscope imaging, and precision-‐automated sample positioning. Electron probe microanalysis (EPM) is based on the principle that a sample will produce x-‐rays characteristic of the elements present in the sample when exposed to a finely focussed electron beam. The electron column and basic instrumentation for EPM are very similar to those from SEM. The major difference is that, while SEM describes the surface morphology by visual magnification of the surface, EPM is able to describe the chemical composition of the sample. A scanning electron microscope, when equipped with an x-‐ray spectrometer, can carry out the same analysis as that of an EPM. Modern scanning electron microscopes are commonly equipped with energy dispersive x-‐ray spectrometers (EDS) that allow qualitative and quantitative chemical analysis. EMP can be used as a powerful analytical tool in coal research if used in conjunction with other analytical techniques such as SEM, transmission electron microscopy (TEM) and optical microscopy, XRD, and Mössbauer spectroscopy (Gupta, 2007).
EPMA equipped with an energy-‐dispersive x-‐ray detector can be used to simultaneously determine the morphology and the constitution elements (within a microscopic size volume) of coal. The procedures for quantitative analysis of coal and elemental contents of coal using SEM and EMPA have been described in detail by Karner and others (nd). However, until recently a major drawback of EPMA was its inability to routinely and precisely analyse samples for the low-‐energy (light) elements such as C, N, O, and F. This was because of 1) the absorption of the long-‐wavelength and low-‐energy X-‐radiation in the specimen, the analyser crystal and the detector window; 2) the low cross-‐sections for ionisation; and 3) spectral interference from higher-‐energy elements. The resulting low count-‐rates and generally low peak-‐to-‐background ratios place a heavy demand on the detection process. Matrix-‐correction programs must be substantially more robust than those for higher-‐energy (heavier) elements (Raudsepp, 1995).
Over the last decade, both the technology and the data evaluation methods of x-‐ray spectra analysis and quantitative determination of sample composition have been improved for EPM. One of the major advances in this field has been the introduction of commercial silicon-‐based spectrometers equipped with thin polymer windows which improve the transmission of low-‐energy x-‐rays, as well as newly designed high resolution energy dispersive detectors such as microcalorimeters. Another important improvement is the development of evaluation models for quantitative analysis, which can handle various types of target samples. Recently, a low-‐energy EPMA and energy-‐dispersive x-‐ray detector with an ultra-‐thin
window was developed allowing quantitative determination of the concentrations of low-‐energy elements, as well as the higher-‐energy elements that can be analysed by conventional EPMA. Furthermore, by varying the excitation voltage (the so-‐called ‘beam variation’ EPMA), useful information on the heterogeneity with respect to chemical composition of individual particles can be obtained (Ro, 2005).
5.2.3 Transmission electron microscopy
A transmission electron microscopy (TEM) utilises energetic electrons to provide topographical, morphologic, compositional and crystallographic information on samples. TEM is a microscopy technique in which a beam of electrons is transmitted through a specimen forming an image in the process. The image is magnified and focussed onto an imaging device, such as a fluorescent screen, on a layer of photographic film, or to be detected by a sensor such as a camera. High resolution TEM (HR–TEM) can be used to deduce the structure of coal at atomic level. The lattice fringe image obtained from HR–TEM provides direct coal or char atomic structural information. TEM with EDS is another method that has been used to identify and characterise the phases of minerals, and to determine the distribution of elements in coal and coal ash (in atomic or weight percentages).
TEM offers the most powerful magnification, potentially over one million times or more. Similar to SEM, TEM has a wide range of applications. It can provide information on surface features, shape, size and structure as well as element and compound structure. It is easy to operate with proper training. Like SEM, TEM is large, expensive, and requires special housing to prevent any possible electric, magnetic or vibration interference. Sample preparation in TEM can be a complex procedure. Glikson (2001) reviewed the TEM technique and its applications including sample preparation, a topic also described by Chen and others (2005). TEM has been extensively applied in studies in morphology and composition of coal and coal ash, especially ultra-‐fine fly ash (Chen and others, 2005; Linak and others, 2006; Silva and da Boit, 2011; van der Merwe and others, 2011; Oliveira and others, 2011; Wu and others, 2013; Damoe and others, 2012).