Mechanism of crystallization of zeolite A microcrystals from initially clear aluminosilicate solution: A population balance analysis

Heating of the clear aluminosilicate solution (33.33 Na2O Al2O3 3.33 SiO2 400 H2O) from the room temperature (23 °C) to that the reaction (40, 50 and 60 °C) causes a spontaneous precipitation of an amorphous aluminosilicate (gel) and simultaneous formation of zeolite nuclei in the gel matrix. Measuring of the mass, ms, of the precipitated amorphous aluminosilicate and simultaneous analysis of the change in the concentrations, CAl, of aluminium and, CSi, of silicon in the liquid phase, showed that aluminium and silicon contained in amorphous aluminosilicate represent only a small fraction of the total amounts of aluminium and silicon in the system. The amorphous aluminosilicate tends to dissolve immediately after the formation, which causes the releasing of nuclei from the gel matrix and their growth by the reaction of monomeric and/or low-molecular aluminate and silicate species from the liquid phase on the surfaces of the growing zeolite crystals. However, the growth process and thus the crystallization of zeolite A do not start immediately after beginning of heating of the reaction mixture (e.g., at the crystallization time tc=0), but later, at a time tc=tg(0)>0. An analysis of the influence of the time of heating of the reaction mixture from the room temperature (23 °C) to the reaction one (40, 50 and 60 °C) have shown that this factor has a negligible influence on the “delaying” of the crystal growth and crystallization. Hence, it was concluded that the “delaying” of crystal growth and crystallization was determined by the time needed for the formation of amorphous aluminosilicate (gel), formation of nuclei in the gel matrix and releasing of the nuclei during the dissolution of the gel matrix. The proposed mechanism of crystallization of zeolite A from clear aluminosilicate solution was confirmed by population balance analysis of the crystallization processes at 40 and 60 °C.