Geochemical fractionation of hazardous elements in fresh and drilled weathered South African coal fly ashes

Abstract

The chemical reactions of dry-disposed ash dump, ingressed oxygen, carbon dioxide, and infiltrating rainwater affect mineralogical transformation, redistribution, and migration of chemical species. Composite samples of weathered coal fly ash taken at various depths and fresh coal fly ash were examined using organic petrographic, X-ray diffraction, X-ray fluorescence techniques, and successive extraction procedures. Results obtained show relative enrichment of glass, Al-Fe-oxides, calcite, and tridymite in the weathered CFA, but the fresh CFA is enriched in mullite, inertinite, maghemite, and ettringite. The enrichment of the weathered CFA in amorphous glass suggests higher reactivity when compared to fresh CFA. The evident depletion of soluble oxides in the weathered CFA is attributed to flushing of the soluble salts by percolating rainwater. Comparative enrichment of examined elements in water-soluble, exchangeable, reducible, and residual fractions of the weathered CFA is partly due to the slow release of adsorbed chemical species from the alumina-silicate matrix and diffusion from the deeper sections of the particles of coal fly ash. Sodium and potassium show enrichment in the oxidisable fraction of fresh CFA.

Physicochemical, Mineralogy, and Thermo-Kinetic Characterisation of Newly Discovered Nigerian Coals under Pyrolysis and Combustion Conditions

“…Furthermore, the highest AC was observed in OME coal, whereas the lowest was observed in AKZ. Typically, ash represents the bulk mineral matter, inorganic or non-combustible residue arising from the coal combustion, which has potential impacts on the environment (Akinyemi et al 2020b). The chemical composition of coal ash is crucial to coal conversion due to its influence on slagging, agglomeration or viscosity of bed materials (Cebeci et al 2002;Özer et al 2017).…”

“…Typically, Si exists as silicon dioxide (SiO2) otherwise termed quartz in coal (Speight 2012), which accounts for 40-90% of the major inorganic components and combustion residues found in coal ash (Wong et al 2020). Quartz is the primary constituent of various granite, quartz, porphyry, and rhyolite rocks and tends to occur due to proximity to coal beds during the process of silicate weathering or coalification (Speight 2012;Akinyemi et al 2020b). Hence, the high Si indicates the presence of SiO2 in OME, which is in good agreement with the high ash content of OME as earlier reported in Table 1.…”

Physicochemical, mineralogy, and thermo-kinetic characterisation of newly discovered Nigerian coals under pyrolysis and combustion conditions

“…Furthermore, the highest AC was observed in OME coal, whereas the lowest was observed in AKZ. Typically, ash represents the bulk mineral matter, inorganic or non-combustible residue arising from the coal combustion, which has potential impacts on the environment (Akinyemi et al 2020b). The analysis of the chemical composition of ash in coal is crucial to the conversion process (Cebeci et al 2002).…”

“…Typically, Si exists as silicon dioxide (SiO 2 ) otherwise termed quartz in coal (Speight 2012), which accounts for 40%-90% of the major inorganic components and combustion residues found in coal ash (Wong et al 2020). Quartz is the primary constituent of various granite, quartz, porphyry, and rhyolite rocks (Speight 2012), and tends to occur due to proximity to coal beds during the process of silicate weathering or coalification (Akinyemi et al 2020b). Hence, the high Si indicates the presence of SiO 2 in OME, which is in good agreement with the high ash content of OME as earlier reported in Table 1.…”

Physicochemical, Mineralogy, and Thermo-Kinetic Characterisation of Newly Discovered Nigerian Coals under Pyrolysis and Combustion Conditions

“…Typically, Si exists as silicon dioxide (SiO2) otherwise termed quartz in coal (Speight 2012), which accounts for 40-90% of the major inorganic components and combustion residues found in coal ash (Wong et al 2020). Quartz is the primary constituent of various granite, quartz, porphyry, and rhyolite rocks (Speight 2012), and tends to occur due to proximity to coal beds during the process of silicate weathering or coalification (Akinyemi et al 2020b). Hence, the high Si indicates the presence of SiO2 in OME, which is in good agreement with the high ash content of OME as earlier reported in Table 1.…”