# Separation of Conospheres from Lignite Fly Ash Using Acetone-Water Mixture
## 3 Results and Discussion
### Chemical Composition  
Table 3 shows the oxide compositions in fly ash and the collected nanospheres characterized by the XRF technique. The sum of SiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\), and Fe\({}_{2}\)O\({}_{3}\) in fly ash was 60.49%, which was in the range of 50%-70%, and the CaO content was higher than 5.0%, hence confirming its classification as class C fly ash (regarding ASTM C618) [40]. Contents of SiO\({}_{2}\) + Al\({}_{2}\)O\({}_{3}\) + Fe\({}_{2}\)O\({}_{3}\) > 50% in high-Ca class C fly ash exhibit cementitious properties that can be used as Portland cement replacement [41]. It should be noted that the chemical composition of cement involves both major and minor oxides, such as fly ash, except for the predominant content of CaO. The presence of high CaO in conjunction with SiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\), and Fe\({}_{2}\)O\({}_{3}\), forming calcium aluminosilicates and aluminoferrite hydrate, leads to the hardening of Portland cement during cement hydration. The chemical composition of cement in comparison to fly ash is available in the literature [42; 43]. Fly ash recovered from the sink part was found to contain those three major components at 61.19%, with other compositions found to be insignificantly different from the fresh fly ash. CaO was not only embedded in the glassy spheres, but was present as free lime in fly ash. The composition of CaO in fly ash usually increases with decreased particle size [44] and has been found to be up to 15-40 wt.% for fly ash produced from lignite coal [45]. The cenospheres have a similar composition to fly ash, but are larger in size. The chemical compositions in the cenospheres were found to mainly consist of the macrocomponents of SiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\), and Fe\({}_{2}\)O\({}_{3}\). The SiO\({}_{2}\) content lay in the range from 36 wt.% to 56 wt.%, the Al\({}_{2}\)O\({}_{3}\) content varied from 16 wt.% to 25 wt.%, and the Fe\({}_{2}\)O\({}_{3}\) content varied in the narrow range of approximately 4 wt.% to 13 wt.%, accommodating the type of cenosphere separated from lignite coal fly ash from the Mae Moh thermal power plant, Thailand, in the magnetic category [46; 47]. The Fe\({}_{2}\)O\({}_{3}\) content in cenospheres of about 7 wt.% involved a magnetic phase based on defect magnetite, while the low Fe\({}_{2}\)O\({}_{3}\) content of 2.5-3.5 wt.% was classified as nonmagnetic cenospheres [31]. It is worth noting that the presence of iron oxide in the aluminosilicate shell of cenospheres in terms of ferrosilite and relatively heavier elements such as Ca, Fe, and Ti, accompanied by a thick shell, could possibly make the cenospheres of density heavier than water, at 1.282 g/cc [24]. As seen in Table 3, for the ratios of 50:50 and 20:80 of acetone/water, the Fe\({}_{2}\)O\({}_{3}\) content in cenospheres tended to increase with the increased water contents. Accordingly, those containing heavy elements in the cenospheres shell enable the cenosphere fractions to be collected in the relatively higher density of the medium (0.90-0.96 g/cc), as the data show in Table 1.  
The cenospheres are a mixture of aluminosilicate mixed with a moderate amount of calcium oxide and ferric oxide and a slight content of S, K, Ti, and trace elements of Ti, Mn, Cr, and Cu. The presence of trace elements demonstrated their amounts in a limited concentration. Variation in the medium properties did not show a significant impact on the reduction of their concentration with a decrease in particle size. Specifically focusing on the oxide iron-aluminosilicate (Fe\({}_{2}\)O\({}_{3}\)-SiO\({}_{2}\)-Al\({}_{2}\)O\({}_{3}\)) system, a sum of Fe\({}_{2}\)O\({}_{3}\), SiO\({}_{2}\), and Al\({}_{2}\)O\({}_{3}\) higher than 80% is regarded as a cenosphere with good quality [23; 48]. As seen in Figure 4a, the sum of the SiO\({}_{2}\) + Al\({}_{2}\)O\({}_{3}\) + Fe\({}_{2}\)O\({}_{3}\) enrichment for the 50:50, 80:20, 100:0 ratios was higher than 80 wt.%.