## 3 Results and Discussion  
### Cenospheres Recovery Yield  
The cenospheres yields obtained from different separation conditions are demonstrated in Figure 1a-e. The effect of mixing methods was particularly investigated by comparing between manual stirring and using ultrasonication. Figure 1a,b shows the nanospheres yield obtained from different collection days (from Day 1 to Day 8) at various fixed fly ash-water ratios for the respective conditions of manual stirring and ultrasonication. For a given collection time, the cenospheres yield tended to increase with decreasing the fly ash content in water; that is, the scheme under low fly ash/water ratios (1:20, 1:40, and 1:80), which correlated with the relatively lower concentration of fly ash (5, 2.5, and 1.25 wt.%, respectively) in the mixtures, is prone to result in a higher recovery yield of cenospheres. The dependence of cenospheres recovery yield on the factor of fly ash/water ratio was more pronounced when incorporating low fly ash, from 1.25 wt.%-5 wt.%, in water medium. This could be clearly observed particularly for the collection time within four days. Decreasing fly ash content to 1.25 wt.% in the mixture led to a high recovery percentage about 4-5 times more than that of the 10-20 wt.% contents. The maximum percent cenosphere recovery achieved was approximately 1 wt.% at Day 1 collection as using the most diluted 1.25 wt.% fly ash content condition. For varying collection time, the plots of cenospheres yield against fly ash content appeared to be similar, especially for Day 1 to Day 4. On further increase in the collection time, the cenospheres yield tended to decrease in comparison. In this study, the mixtures were repeatedly stirred before leaving them to settle and the floaters were collected at each time. The recovery of the floaters was improved by a repeated stirring procedure [44]. While the longer collection period resulted in the relatively lower amount of cenospheres to be obtained. The graphs shown in Figure 1a,b comparing the conditions with and without ultrasonication are approximately in the similar pattern. The effect of size classification for the starting fly ash on the cenospheres recovery yield is shown in Figure 1c,d; the weight fraction in Figure 1e is schematically demonstrated as an alternative view. It can be seen that the cenospheres collected from the small sized fractions (<45 and 45-105 \(\upmu\)m) has higher yield than those collected from the relatively larger sized fractions (106-250 and >250 \(\upmu\)m). This result has suggested that majority of Mie Moh nanospheres, > 80 wt.%, predominantly contained small particles, with a size of < 105 \(\upmu\)m.  
Figure 1f shows the accumulative plots of percent cenospheres recovery against the ash content. The accumulative cenosphere yields are in the range of -0.8%-3.5%. These values were found in the similar ranges reported for various power stations [16; 18; 29]. The cenospheres recovered from fly ash contains predominantly cenospheres, porous char, individual ferrospheres, and ferrispheres [13]. These are the common characteristics of cenospheres, generally observed, and could to some extent contribute their weight fraction in the percent yield. The variation of cenosphere yield was believed to relate to the coal type, the coal mineralogy, and the combustion conditions [16]. From the overlay graphs (Figure 1f), it could be summarized that the separation efficiency is strongly dependent upon fly ash content. The cenospheres yield tended to increase for the more diluted concentrations, as the ascending curves seen when incorporating low ash concentrations. This is probably due to the effect of drag force (inter-particle interaction) governing the lateral segregation of light and heavy particles, thus enhancing the velocity of separation. The drag force leads to a formation of fingering flow structure along the gravitational direction in a sufficient solid concentration [18].