5. Jet loop assisted pre-synthesis
5.4. Synthesis of zeolite X from the jet loop and cured filtrate
Table 5-5: Comparison of silicon extraction
Process % Silicon
extracted Reference
Fusion 26 This study
Jet loop and curing 31 This study
Fusion 32 Ojumu et al., (2016)
Ultrasonictaion 24 Ojumu et al., (2016)
The next section will deal with the attempt to synthesize zeolite X from the filtrate obtained after the jet loop and curing process using the optimum conditions found within this section. There would need to be adjustments to the Si/Al ratio of the designated filtrate in order to achieve a similar ratio of Si and Al as was found in the filtrate produced with the fusion process prepared in Chapter 4.
Figure 5-13: XRD spectra of hydrothermal synthesis of jet loop and cured filtrate for 8 hours at 900C at varying Al(OH)3 addition: JLZ25 = 0.25 g; JLZ50 = 0.5 g; JLZ75 = 0.75 g (S = sodalite)
As can be seen from the XRD spectra in Figure 5-13, regardless of the calculated Al addition mentioned previously the only phase achieved was that of the sodalite zeolite. As sodalite is a denser phase than zeolite X it may be due to Ostwald’s rule of ripening; thus the reaction time needs to be reduced in order to target zeolite X, if it is at all possible to target it through changing of the physical parameters. Thus the calculated value of 0.5 g of Al(OH)3 addition was held constant while the synthesis time was reduced from 8 hours to 6, 4 and 2 hours, keeping the synthesis temperature at 900C, and XRD results are presented in Figure 5-14.
Figure 5-14: XRD spectra of varying the hydrothermal synthesis time of jet loop and cured filtrate at 900C with 0.5 g of Al(OH)3 addition
Figure 5-14 shows that reducing the hydrothermal synthesis time down to as low as 2 hours did not favour the formation of zeolite X, again sodalite was the only phase that was achieved. This would demonstrate that varying only the physical parameters does not have an effect when trying to obtain the zeolite X. Thus in order to gain a better insight into the molar composition of the filtrate compared to that of the fusion filtrate as well as two filtrates from other studies that synthesized hierarchical zeolite X, the molar regimes were calculated for all the differing studies. To have an understanding of how these molar regimes differ, a ternary diagram was constructed to represent the molar regimes of these studies pictorially and is presented in Figure 5-15.
Figure 5-15: Ternary plot of the molar regimes in differing studies: JL = Jet loop and curing in this study; F = Fusion method in this study; N = Musyoka, (2012); ML = Cornelius, (2015); JLZ25, JLZ50, JLZ75 = Sodalite zeolites formed through Al(OH)3 addition to jet loop and cured filtrate
Looking at the ternary diagram in Figure 5-15, it can be seen that the molar concentration of silicon within the filtrate obtained from the jet loop and curing process was higher than the other molar compositions used to synthesize zeolite X. However, what is also noted is that the other routes where zeolite X was successfully synthesised had higher concentrations of sodium than found in the filtrates obtained from the jet loop and curing process. Also seen on the ternary plot are the molar formulations of the sodalite zeolites achieved through hydrothermal synthesis of the jet loop filtrate with Al(OH)3 addition as explained previously. This shows that adjustment of the Si/Al ratio could not target the hierarchical zeolite X. Therefore, additional NaOH may need to be added to the jet loop and curing process. Thus, it may be advisable to change the mass ratio of 1:1:5 of CFA:NaOH:H2O that is initially fed into the jet loop mixing process to allow for the excess sodium that is required to synthesis zeolite X. This is due to the fact that in all the other studies using the fusion pre-synthesis process the mass ratio used to fuse the fly ash with NaOH is that of 1:1.2 of CFA:NaOH compared to the 1:1 of CFA:NaOH used in the jet loop and curing process. This optimisation however, did not form a part of this study. It would be advisable to optimise this mass ratio in the jet loop as it can also be seen by looking at the molar regimes of the various studies in Table 5-6 that there was also a
higher water content in the synthesis mixtures. Therefore, there may be a need to adjust the water content as well. It is also highlighted that in the all the studies where hierarchical zeolite X was synthesised, ultra-pure water was used while when using the jet loop only tap water was used as a solvent, which could contain contaminants inhibiting the formation of zeolite X.
A material balance was not performed around the hydrothermal section of the process because as before the yields of the zeolite were less than a gram in all instances and would again have precluded an accurate balance around the process. What is needed is to optimize the process for greater yields of product which would allow for elemental analysis and an accurate material balance.
Table 5-6: Comparison of molar formulations
Study* Al Na Si H2O
Musyoka (2012) 0.07 5.43 1.00 123
Cornelius (2015) 0.12 14.6 1.00 163
Fusion in this study 0.10 3.69 1.00 95.33 Jet loop and cured
in this study 0.04 3.34 1.00 79.63 *Normalized to Si