Chemical composition of the fly ashes

Chemical composition of fly ash has a significant influence on both its potential applications and on the environmental impact of its subsequent use. The XRF results of the chemical/elemental composition of the fly ashes used in this study are depicted in Table 4.1. The results obtained give fundamental information on the major and trace elemental composition of the fly ash matrix. The analysis was done in duplicate in order to minimise the error in the results obtained.

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Table 4-1: Major elemental composition of coal fly ashes used in this study

Major Oxides Arnot Tutuka Hendrina Lethabo Matla

(Mean Mass %) Batch 1 Batch 2

SiO2 55.66 55.44 52.63 49.79 58.32 58.44 Al2O3 27.95 31.51 26.49 31.75 31.36 31.25 Fe2O3 3.22 4.94 4.87 3.17 3.04 3.09 MnO 0.04 0.03 0.05 0.00 0.02 0.02 MgO 1.91 1.18 1.31 0.98 1.13 1.14 CaO 4.38 3.76 5.33 4.62 3.16 3.21 Na2O 0.31 0.04 0.55 0.09 0.46 0.46 K2O 0.45 0.47 0.82 0.63 0.54 0.54 TiO2 1.13 1.11 1.46 0.67 1.16 1.17 P2O5 0.26 0.30 0.38 1.46 0.39 0.40 SO3 0.03 0.06 0.06 0.23 0.02 0.02 Loss On Ignition 4.74 1.22 6.09 6.59 0.40 0.28 Sum (%) 100.07 100.07 100.04 99.98 100.00 100.00 SiO2/Al2O3 1.99 1.76 1.99 1.57 1.86 1.87

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Table 4-2: Trace elements in the coal fly ashes used in this study

Trace elements Arnot Tutuka Hendrina Lethabo Matla

(ppm) Batch 1 Batch 2 As 49.10 - 85.70 - 41.09 41.09 Ba 708.93 485.7 913.11 - 918.23 917.53 Ce 128.45 254.0 155.80 - 112.73 107.75 Co 43.87 30 39.52 12.55 14.84 17.97 Cu 46.88 109.9 51.43 51.03 53.85 55.71 Nb 58.10 37.2 65.92 39.67 53.05 57.66 Ni 24.40 124.9 22.04 58.12 21.95 20.45 Pb 56.56 90.3 48.83 65.37 51.71 51.81 Rb 29.29 56.2 41.01 31.18 27.57 35.77 Sr 1011.45 988.9 1384.72 1194.85 970.66 976.19 V 113.49 78.6 122.29 85.12 123.49 110.70 Y 84.78 93.8 98.29 15.31 73.72 71.71 Zn 42.09 135.3 38.03 93.69 39.54 39.48 Zr 442.60 - 492.99 424.09 385.24 384.58 Mo 11.28 - 7.48 - 7.86 10.58 Th 384.21 - 558.96 - 372.75 373.09

The XRF results in Table 4.1 revealed that the glass phase in all the fly ashes used in this study accounts for more than 60 to 80 % of the fly ash and this indicated that South African FA would supply a readily available source of Si and Al for zeolite synthesis. The average SiO2/Al2O3 ratio of the fly ashes in this study was found to be 1.99, 1.76, 1.99, 1.57, 1.86 and

1.87 for Arnot batch 1, Arnot batch 2, Tutuka, Hendrina, Lethabo and Matla fly ashes, respectively. This ratio is appropriate for the synthesis of low Si-zeolites with high cation exchange capacity (Querol et al., 1996). This ratio plays an important role during zeolite synthesis since it places constraints on the framework composition of the zeolite produced (Szostak, 1989). Changing this ratio may result in a change in the final structure obtained and may also lead to the crystallisation of unwanted phases (Szostak, 1989). Although the chemical compositions of the two batches of Arnot fly ash were similar, these samples exhibited a different SiO2/Al2O3 ratio i.e. 1.99 and 1.76 for batch 1 and batch 2 respectively.

52 These South African fly ashes are classified as class F type (according to the ASTM standard C618) because their SiO2 + Al2O3 + Fe2O3 content was assessed to be greater than

70 %. This type of fly ash results from the burning of the harder, older anthracite and bituminous coal (Vassilev and Vassileva, 2007). These fly ashes were also found to contain low content of CaO and MgO. These oxides play a significant role during zeolite synthesis. Ca2+ and Mg2+ were reported to act as competing ions during zeolite synthesis. Ca2+ was also reported to have structure breaking properties which lead to the interference with the zeolite crystallisation process (Catalfamo et al., 1993). It was also reported that Ca-bearing phases can act as an inhibitor for the synthesis of zeolites through the formation of Calcium Silicates (Ríos et al., 2009; Juan et al., 2007). Fe-bearing minerals, such as magnetite, were also found to show an inert behaviour during zeolite synthesis (Juan et al., 2007). Thus the lower the content of Ca and Fe bearing minerals, the less the interference during the crystallisation of zeolites. Nonetheless, all the fly ashes used in this study contained low content of Fe-bearing minerals ranging between 3 to 5 %.

The fly ashes used in this study contained low contents of sodium (Na), as can be seen in Table 4.1. Sodium plays a crucial role during zeolite synthesis as a charge-balancing cation. Therefore, most of the sodium available during the synthesis of zeolites in this study came from the NaOH solution and the sodium aluminate source used, since the fly ash contained low concentration of this element. The sulphur trioxide (SO3) content in the fly ashes used in

this study was low (ranging between 0.02 to 0.06 %). This may be due to the fact that these fly ashes were produced from low sulphur containing coals. However, the Hendrina FA showed a high SO3 content. Although low sulphur coals may have been used, the content of

SO3 in the FA may increase due to the injection of SO3 in the electrostatic precipitator.

Therefore, FA collected from electrostatic precipitators may contain high amounts of SO3.

Hendrina FA might have been collected from the electrostatic precipitators, resulting in a high SO3 content.

The Loss-On-Ignition (LOI) measures the amount of unburned carbon remaining in the fly ash and it is an important chemical parameter, since it can be used as a screening tool for fly ash for use in concrete and cement manufacturing and also assists in the prediction of the quality of zeolites synthesised from fly ash. Rayalu et al. (2001) pointed out that large amounts of unburned carbon (high values of LOI) interferes with the fusion step, thereby affecting the quality of the fused product. Compared to the other fly ashes, Hendrina fly ash had the largest amount of unburned carbon (6.59 %) followed by Tutuka fly ash (6.09 %) then batch 1 of Arnot fly ash (4.74 %). It was noticeable that Arnot batch 1 and 2 fly ashes differed considerably in terms of the LOI (i.e. 4.74 % for batch 1 and 1.22 % for batch 2). This indicated that the power station burner conditions were altered during the time the samples were taken.

53 These fly ashes were also found to contain some potentially toxic trace elements such as As, Pb and Ba (Table 4.2). Amongst the dominant trace elements present in the fly ash, Sr, Ba, Ce, Zr and Th were found to be in significantly higher concentration relative to the other trace elements in all the fly ashes. It is important to highlight that although other trace elements such as As, Ba and Ce in Hendrina fly ash were not indicated in the table, it does not mean that this fly ash did not contain these elements. These elements were not analysed due to experimental constraints. Strontium (Sr) was found to be the highest trace element in all the fly ashes with concentrations between 970 to 1384 ppm (Table 4.2) being observed. Similarly, Barium (Ba) also existed in high concentrations, especially in Tutuka (913.11 ppm), Lethabo (918.23 ppm) and Matla (917.53 ppm) fly ashes. Ba concentrations ranged between 485 and 919 ppm. Fly ashes used in this study were also found to contain large amounts of zirconium (Zr) and thorium (Th) with concentrations ranging between 384 to 493 ppm and 372 to 559 ppm respectively. Nevertheless, none of the abovementioned trace elements were ever reported to affected zeolite synthesis. However, they may be detrimental to the environment.