Crystallization mechanism of zeolite A from coal kaolin using a two-step method

Zeolite A was synthesized from coal kaolin using a two-step method. The crystallization mechanism was studied. The synthesis was composed of pre-crystallization at a low temperature and crystallization at a high temperature. The influence factors on crystallinity were investigated, including calcination temperature of kaolin, crystallization time, pre-crystallization temperature and time, and composition of synthetic liquid. X-ray diffraction (XRD), scanning electron microscopy (SEM) and infrared spectroscopy (IR) were used to characterize the nucleation and crystal growth processes. The results indicate that abundant nuclei were formed during the pre-crystallization period, whereas crystallization process during the first h in crystallization period was still a nucleation stage. Also, metakaolin had dissolved thoroughly after the synthesis liquid crystallized for 3 h, and the nuclei were rapidly grown into zeolite A crystals at this time. An excess crystallization time results in a dissolution of as-synthesized crystals and a decrease of crystallinity as well as a too high concentration of OH−.