## 3 Results and Discussion
### The particle Size Distribution  
As stated earlier, we used a Laser diffraction analyzer for measuring the particle size distributions of the size-fractionated fly ashes. A summary of the results obtained from the magnetic and nonmagnetic fractions is available in Figure 1 and Table 2. It can be seen from Table 2 that, more than 85% of the fly ash particles of UB and PL samples are smaller than 90 \(\upmu\)m. Considering the results of sieve grading tests of the feed coal, an appreciable change was shown in the particle size. For a desirable burning efficiency in Pulverised Fuel Combustors, 75 \(\upmu\)m is determined as the maximum size of the particle for the feed coal [38]. In the present case, however, larger coal particles percentage is significant, leading to inefficient combustion. Large coal particles in feed coal indicate another implication. Once the larger coal particles underwent devolatilization and during the burning of the char, will leave a noticeable mineral matter. Such outputs were quite large, meaning that these particles have been left out of further melting and fragmentation/transformation. It can be concluded that such a sieve analysis of the feed coal gives new insight into ash formation.  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Element (\%)**} & \multicolumn{2}{c}{**Content (\%)**} \\ \cline{2-3}  & \multicolumn{2}{c}{**Uong Bi (UB) Fly Ash**} & \multicolumn{2}{c}{**Pha Lai (PL) Fly Ash**} \\ \cline{2-3}  & **Fly ash** & **Magnetic** & **Nonmagnetic** & **Fly ash** & **Magnetic** & **Nonmagnetic** \\ \hline \(>\)250 & 0.41 & 0.14 & 0.27 & 0.33 & 0.12 & 0.21 \\
90–250 & 16.48 & 2.03 & 14.45 & 14.48 & 2.59 & 11.89 \\
45–90 & 25.58 & 3.52 & 22.06 & 31.19 & 4.62 & 26.57 \\
32–45 & 31.18 & 2.43 & 28.75 & 36.66 & 3.23 & 33.43 \\ \(<\)32 & 26.24 & 2.11 & 24.13 & 16.93 & 2.51 & 14.42 \\
**Total** & **99.89** & **10.23** & **89.66** & **99.59** & **13.07** & **86.52** \\ \hline \hline \end{tabular}
\end{table}
Table 2: Magnetic and nonmagnetic contents in different size fractions of UB and PL fly ashes.  
According to weight percentage distribution, no certain correlation exists between the weight percentage and the size of the particles. An earlier communication has reported an exact analysis for the size of the particle of diverse size fractions such as the magnetic and non-magnetic ones and their correlations with the utility and combustion [26]. Overall, the reports indicate the larger particles are less useful for partial replacement of cement in concrete. Other researchers have also investigated the effect of particle size on the chemical composition of fly ash [17; 32; 33]. Those elements that display the highest concentration relevancy on particle size are mainly connected with an elemental form that sublime or boils at coal combustion temperature [11; 15; 37; 39]. It is important to note that the Si content increases as the size of the particles decreases. This trend is apparently due to the fact that the carbon content decreases as the size of the particles decreases. Hence, it is expected that these fractions will be quite rich with Si. The rise in the Fe content in tandem with a reduction in the particle size is also fairly inconspicuous. These observations help us conclude that reduced carbon content in tandem with decreased particle size is mainly compensated by silica.  
The magnetization curve of samples particles illustrated in Figure 2. The superparamagnetic properties for samples obviously can be observed. The saturation magnetization obtains for PL sample is 12.1 emu/g, and for UB sample is 4.2 emu/g. Therefore, it can be concluded the magnetite concentration in the PL sample is more than UB sample.  
Figure 1: Particle size distribution for the magnetic fraction of UB and PL fly ashes.