**2.4. Scanning Electron Microscopy and Electron Dispersive X-ray Spectroscopy (SEM/EDS)**

Microstructural and chemical composition investigations of coal and coal ash were carried out by scanning electron microscopy/electron dispersive x-ray spectroscopy (SEM/EDS). For SEM/EDS aluminium stubs were coated with carbon glue; when the glue was dry, but still sticky; a small amount of powder residue samples was sprinkled onto the stub. The excess residue sample powder was tapped off and the glue allowed complete drying. The residue samples were then coated with carbon in an evaporation coater and were ready for analysis with the SEM. The SEM is an FEI Nova NanoSEM (Model: Nova NanoSEM 230); The EDS analyses were determined at 20 Kv and 5 mm working distance. The EDS detector is an Oxford X-Max (large area silicon drift detector) using the software program INCA-(INCAmicaF+ electronics and INCA Feature particle analysis software).

#### **2.5. Transmission Electron Microscopy (TEM)**

Pulverized sample (~1-2) g of the coal and coal ash samples was poured into a small conical container while little amount of ethanol was added to the sample to serve as medium for solution. This solution was then placed inside the centrifuge for few minutes (5 mins), drops of the stirred solution was then placed on a labelled 200 μm and 400 μm of copper grid underlain by a filter paper with a hot lamp light focused directly on the samples to dry up the earlier added ethanol. The resultant mixture was placed inside the air gun channel so as to project the beam on it for image analysis at a nanometric scale. The TEM analysis of study was carried out on TECHNAI G2 F20 X-TWIN MAT 200Kv field emission transmission electron microscopy.

Mineralogy and Geochemistry of Sub-Bituminous Coal and Its Combustion Products from Mpumalanga Province, South Africa 51

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All of the water is lost during the high-temperature ashing. Pyrite is the main species of sulfur oxidation in the coal samples studied. Pyrite occurs as typical syngenetic framboidal, euhedral and massive cell filling forms [20]. This mineral shows a highly inhomogeneous distribution in the coal samples. Pyrite is probably the most environmentally interesting mineral in the run of mine and beneficiation of coals and their generated wastes because of

Pyrite is transformed to hematite and sulfur dioxide during coal incineration at 815°C [20]. Some of the sulfur dioxide may remain combined with calcium in the ash, but much is lost [21]. The weathering of pyrite produces acid conditions that may leach trace elements associated with the pyrite and other constituents in the coal [22]. Quartz is the most common mineral in the coal samples studied. This mineral has mostly detrital genesis [23]. The shape of the quartz grains is rounded to semi-rounded, indicating an intensive transport before their deposition in the basin. The content of quartz is also high in the coal fly ash because this mineral is commonly stable/inert at combustion conditions (Fig. 3).

21742190

0 500 1000 1500 2000 2500 3000 3500 4000 4500

Wavenumber (cm-1)

The mineralogical analysis by depth of the core ash samples was carried out with X- ray diffraction technique (XRD). This is to better understand the mineralogical changes (i.e. secondary phases) under the real dry disposal conditions. The experimental protocol for this section is presented in section 2.3. The results of the XRD analysis of samples of the drilled weathered dry disposed fly ash aged 2 week, 1 year and 20-year-old showed quartz (SiO2) and mullite (3Al2O3·2SiO2) as main crystalline mineral phases (Fig. 3). Other minor mineral phases identified are; hematite (Fe2O3), calcite (CaCO3), lime (CaO), anorthite (CaAl2Si2O8), mica (Ca(Mg,Al)3(Al3Si)O10(OH)2) and enstatite (Mg2Si2O6). This is in general agreement with

its propensity to oxidize during weathering and production of sulphuric acid.

1625

1450

684

746

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529

413

**Figure 2.** Infrared (FTIR) spectra of coal sample


0.0

0.2

Absorbance (A)

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**3.2. Mineralogy of weathered drilled ash cores** 

#### **2.6. Proximate and ultimate analyses**

Proximate and ultimate analyses were performed on coal samples based on ASTM Standards [12]. All runs were repeated to check the instrument's results repeatability and reproducibility.
