Chemical, physical and morphological changes in weathered brine slurried coal fly ash
Energy production from coal comes with an environmental cost because of the toxic waste produced during coal combustion such as coal ash and brine which are potential water and soil pollutants. Coal ash and brine contain toxic elements which can leach and contaminate soils and ground water if not properly disposed. This study investigated the mobility of species in coal fly ash co-disposed with brine at Sasol Secunda power station in order to establish if the ash dam could act as a salt sink. The ash was dumped as a slurry with 5:1 brine/ash ratio and the dam was in operation for 20 years. It was hypothesized that the disposed Secunda fly ash was capable of leaching toxic metal elements into the surrounding soils and ground water and therefore could not be used as a long term sustainable salt sink. Weathered fly ash samples were collected along a 51 m depth core at the Secunda ash dam by drilling and sampling the ash at 1.5 m depth intervals. A fresh fly ash sample was collected from the hoppers in the ash collection system at the power station. Characterization of both Secunda fresh ash and Secunda weathered ash core samples was done using X-ray diffraction (XRD) for mineralogy, X-ray fluorescence (XRF) for chemical composition and scanning electron microscopy (SEM) for morphology. Analysis of extracted pore water and moisture content determination of Secunda fresh ash and Secunda weathered ash core samples was done in order to evaluate the physico-chemical properties of the fly ash. The chemical partitioning and mobility of metal species in the ash dam was evaluated using the sequential extraction procedure. The XRD spectra revealed quartz, mullite and calcite as the dominant mineral phases in the weathered Secunda ash core samples while Secunda fresh ash contained quartz, mullite and lime. The major oxides identified by XRF analysis for both Secunda fresh ash and Secunda weathered ash include: SiO₂, Al₂O₃, CaO, Fe₂O₃, MgO, Na₂O, TiO₂ and K₂O. The minor oxides identified for both Secunda fresh ash and Secunda weathered ash were P₂O₅, SO₃ and MnO. The trace elements identified for both Secunda fresh ash and Secunda weathered ash were As, Ba, Ce, Co, Nb, Ni, Pb, Rb, Sr, V, Y, Zr and Th. However, U was detected in some of the Secunda weathered ash samples but not in Secunda fresh ash. Both Secunda fresh ash and Secunda weathered ash was classified as class F based on the sum of the oxides of silicon, aluminium and iron by mass and the CaO content as reported by XRF analysis, and further classified as sialic and ferrocalsialic type highlighting the significant levels of Si, Al, Ca and Fe in the fly ash based on XRF analysis. The XRF analysis further showed that brine codisposal on the ash may have been responsible for the slight enrichment of some species such as Na, SO₄²⁻, Mg, K and V in the disposed Secunda weathered fly ash. However, there was no significant accumulation of these species in the disposed fly ash despite continuous addition of an estimated 117.65 billion litres of brine over the 20 year period that the dam existed. Furthermore, Secunda ash dam showed an overall total salt capture capacity of only -0.01 weight %, a strong indication that the ash dam was incapable of holding salts and would release elements to the environment over time. The scanning electron microscopy (SEM) analysis revealed spherical particles with smooth outer surfaces for Secunda fresh ash while Secunda ash core samples consisted of agglomerated, irregular particles appearing to be encrusted, etched and corroded showing that weathering and leaching had occurred in the ash dam. A decrease in pH, electrical conductivity (EC) and total dissolved solids (TDS) was observed in Secunda ash core samples compared to Secunda fresh ash. While Secunda fresh ash (n = 3) had a pH of 12.38 ± 0.15, EC value of 4.98 ± 0.03 mS/cm and TDS value of 2.68 ± 0.03 g/L, the pH of Secunda ash core (n = 35) was 10.04 ± 0.50, the EC value was 1.08 ± 0.14 mS/cm and the TDS value was 0.64 ± 0.08 g/L. The lower pH in the ash dam created an environment conducive to the release of species through leaching, while the lower EC and TDS in the ash dam implied the loss of ionic species from the ash which resulted from leaching. The moisture content (MC) analysis indicated that Secunda ash dam was very damp with an average MC of 54.2 ± 12.66 % for Secunda ash core creating favourable conditions for leaching of species in the ash dam while Secunda fresh ash had MC of 1.8 ± 0.11 %. The bottom of Secunda ash dam appeared water logged which could cause slumping of the dam. The sequential extraction procedure revealed that the major and trace elements contained in both Secunda fresh fly ash and Secunda weathered fly ash could leach upon exposure to different environmental conditions. The elements showed partitioning between five geochemical phases i.e. water soluble fraction, exchangeable fraction, carbonate fraction, Fe & Mn fraction and residual fraction. The labile phases consisted of the water soluble fraction, exchangeable fraction and carbonate fraction. The % leached out in the labile phases was expressed as a fraction of each element‟s total content e.g. Si (6.15 %) meant that 6.15 % of the total amount of Si in the ash was released in the labile phases. Na was the most labile among the major elements in the ash dam while Si and Al which form the major aluminosilicate ash matrix also showed significant lability. The % leached out in the labile phases for these major elements was as follows: for Secunda fresh ash: Si (6.15 %), Al (7.84 %), Na (11.31 %); for weathered Secunda ash core samples (n = 35): Si (7.53 %), Al (8.12 %), Na (11.63 %). This study showed that the fly ash generated at Sasol Secunda power station could not be used as a long term sustainable salt sink. The wet disposal method used at Sasol Secunda power station poses a high risk of groundwater contamination due to the high liquid to solid ratio used to transport the ash for disposal, which may lead to rapid dissolution of all the soluble components in the fly ash. The large volumes of brine that pass through Secunda ash dam in the wet ash handling system present a greater environmental concern than the dry ash handling system which involves small amounts of brine entering the ash dump.