Conventional versus alkali fusion synthesis of zeolite A from low grade kaolin

Abstract

Synthesis of zeolite A using low grade Bombowha kaolin from Ethiopia has been carried out by two methods: the conventional hydrothermal synthesis and alkali fusion followed by hydrothermal treatment. In the synthesis process, the metakaolinization was achieved by calcination of the kaolin at 600°C for 3h for the conventional method and for the alkali fusion method, the activation of the raw kaolin was done by alkali fusion for only 1h. Zeolite A was the main product obtained using 3M NaOH concentration, 100°C reaction temperature and 3h crystallization time in both methods. The molar gel composition for the best synthesis conditions for both methods was Na2O: Al2O3: 2SiO2: 37H2O. The main advantage of the alkali fusion method is that it allows using a low grade virgin kaolin without purification, whereas the conventional method only yields good quality zeolite A when the raw kaolin is purified. The starting raw kaolin, metakaolin and synthetic products were characterized by X-ray diffraction (XRD), ICP-OES elemental analyses, Scanning Electron Microscopy (SEM), Thermogravimetric Analysis (TGA) and Cation Exchange Capacity (CEC). Cubic crystals with rounded edges of zeolite A having crystallinity of about 85% and Cation Exchange Capacity of 300mg CaCO3/g of anhydrous zeolite was achieved by the alkali fusion method of synthesis using raw, virgin Bombowha kaolin.