Efficient and sustainable synthesis of zeolite NaA from Bangka kaolin: A study of reused filtrate as partial nutrients for the next synthesis

Abstract

The synthesis of zeolite NaA from Bangka kaolin was started by activation step with calcination at temperature 650°C for 2 hours. This activation produced metakaolin, an amorphous phase as assigned by XRD pattern. Based on the result by XRF technique, metakaolin produced from thermal treatment was mostly composed of silicon and aluminum with SiO2/Al2O3 ratio=1.63, and other minor metal elements. This metakaolin was suitable to use as a source for zeolite NaA synthesis which had Si/Al ratio close to one. For the synthesis of zeolite NaA, metakaolin was mixed with NaOH solution, followed by crystallization at 100°C for 24 hrs. The solid product, zeolite NaA was separated by filtration. The resulted filtrate was reused for next zeolite NaA synthesis by the addition of metakaolin. The synthesis was performed by varying initial NaOH concentrations of 2, 3, 4, 5, and 6 M at a constant crystallization temperature at 100°C and constant crystallization time of 24 hours. The synthesis products were characterized by FTIR spectroscopy, X-ray diffraction, and scanning electron microscopy technique. The optimum results with the highest degree of purity and structural order were obtained from initial NaOH concentration 3M. Synthesis with NaOH concentration higher than 3M produced hydroxisodalite as another phase. The reused of filtrate in the synthesis showed the good product of zeolite NaA up to two times reused. Further reused of filtrate resulted significant decrease of purity and structural order. Further investigation indicates that another factor such as aging time for three days before synthesis could produce zeolite NaA at lower crystallization temperatures. While the amount of metakaolin in the initial mixture did not give a significantly better product. The production of zeolite NaA from Bangka kaolin using reused filtrate as partial nutrients could be an excellent way to minimize environmental impacts and decrease processing costs.