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Influence of alkalinity during the precipitation of gel on the nucleation of zeolite A

A series of aluminosilicate gels having the molar oxide composition: (3.31 + x) Na2OxAl2O3x2 SiO2x333.33 H2O, where x = 0, 0.069, 1.38, 2.07 and 2.763 were prepared by adding 50 ml of sodium silicate solution [(1.03 + x) Na2OxSiO2x55.55 H2O] into 100 ml of stirred sodium aluminate solution [1.25 Na2Ox Al2O3x222.22 H2O]. Before mixing, the solutions were preheated to the reaction temperature (80o C). After the precipitation was finished, each gel was diluted with 50 ml of hot (80o C) NaOH solution of appropriate concentration, so that the final batch composition was: 6 Na2OxAl2O3x2 SiO2x444.44 H2O. The gels prepared in the above mentioned way were stirred at 80o C until amorphous phase has completely been transformed to zeolite A. Kinetics of crystallization were studied by different methods such as: x-ray diffractometry, optical and scanning-electron microscopy, particle size analysis and chemical analysis of of the solid and liquid phase. Experimental results were analyzed and discussed in the terms of the influence of OH- ions on the formation of gels and on the number and distribution of nuclei in the gel phase and thus on the rate of nucleation of zeolite A from the gels prepared under different alkalinities.

zeolite A; alkalinity; alumosilicate gel; nucleation; crystallization

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