3.2. Zeolite synthesis
Table 4 shows the results of zeolite synthesis under the different
conditions studied. This table displays the characterization of
the products (XRD analysis, SEM, CEC) and an estimate of zeolite
content in the synthesized material through Eq. (1). In the first syntheses
(Tests 1–13) both ashes in reactor A were evaluated at
150 C. For all the tests the use of 1.0 mol L1 NaOH resulted in
greater yields, reaching 50% and 60% for UA and UB ash, respectively.
These tests also indicated that alkali concentration exerts
a more significant influence on zeolite formation when synthesis
is performed with raw ash. This behavior was more evident for
UB ash, with a 16% increase (Tests 6 and 7) in raw ash and only
4% (Tests 9 and 10) in the fine fraction. A further increase in alkali
concentration (3.5 mol L1) did not lead to Na-P1 formation, but
rather to undesired zeolitic phases (cancrinite, sodalite and X).
These zeolites exhibit lower CEC values compared to Na-P1 and
are not suitable for adsorption processes. The chemical composition
of raw ash and its respective fine fractions, displayed in
Table 1, show no significant differences in term of total Al and Si
content, whereas the molar Si/Al ratio varies from 2.9 to 2.5, with
higher values for UA ash. However, different values are observed
for amorphous Si and Al content (Table 2), which are effectively
dissolved during the first phase of zeolitization. Raw and fine UB
ash fractions exhibit 82% and 79% of amorphous Si and Al oxides,
respectively. These values are higher than those recorded for UA
ash (73%). These results may justify the lower yields observed for
the UB fine fraction and UA raw fraction when compared to UB
raw ash. Table 3 shows a significant reduction in unburned
material in fine particles, although absolute values are too low
(0.27–0.41%) to influence the reaction. Magnetic material content
in UA (5.1%) and UB (3.3%) raw ash is consistent with the larger
yields observed in zeolites synthesized from the latter. Lower reactivity
of the UB fine fraction, significantly enriched in magnetic
material (9.8%), reinforces the possible influence of this parameter
in zeolitization. However, testing conducted on ash (Tests 5 and
11), from which the magnetic material is previously removed,
resulted in negligible yield increases (62%). The addition of crystallization
seeds (pure Na-P1) into the reaction mixture was tested