The present work deals with the hydrothermal synthesis of Na zeolites (Na-A, Na-X and Na-P) and hydroxysodalite using kaolinite calcined at 650°C as starting material. The focus was on definition of the most favourable conditions for the synthesis of zeolite Na-A and Na-X from metakaolin in order to economize on both energy (i.e. synthesis temperatures) and reaction time and to enlarge the field of pure and isolated synthesized phases. Metakaolin was mixed with calculated amounts of NaOH solution and sodium silicate and five sets of experiments were carried out at ambient pressure and 68±0.1°C varying the SiO2/Al2O3 ratio from 2.2 to 7. Optimal conditions for crystallization of Na-A zeolite from kaolinite were reached with a SiO2/Al2O3 ratio of 2.2 plus 4 M NaOH without adding sodium silicate; transformation into hydroxysodalite develops after ∼8 h. For SiO2/Al2O3 ratios between 4 and 7, crystallization of the separate Na-X zeolite phase could be achieved and transformation into Na-P and hydroxysodalite occurred after 382 h and 190 h, respectively. For SiO2/Al2O3 ratios between 5 and 6, transformation of metakaolin into Na-X plus Na-A, hydroxysodalite and Na-P occurred, and the field within which Na-A and Na-X zeolite exists overlapped that of the other zeolites.
The products of synthesis were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), inductively coupled plasma optical emission spectrometry (ICP-OES), infrared spectroscopy (IR) and thermal analyses (TG-DTG-DTA).
Obtaining pure Na-A and Na-X zeolite from kaolinite treated at low metakaolinitization temperature (650°C) and low hydrothermal synthesis temperature (68°C) represents a considerable economic advantage in terms of both energy and time.