Preparation of granulated LTA and SOD zeolites from mechanically activated mixtures of metakaolin and sodium hydroxide

The LTA and SOD zeolites were synthesized from the mechanically activated solid mixtures of metakaolin, NaOH and γ-Al2O3. The process of synthesis is controlled by X-ray diffraction, IR-spectroscopy, and scanning electron microscopy. The stages of both thermal treatment and hydrothermal crystallization were investigated and optimized. The size of LTA particle was directly measured by scanning electron microscopy as well as calculated using the X-ray diffraction spectroscopy data. It was shown that the maximum content of LTA can be achieved with an γ-Al2O3 excess over the stoichiometry of the synthesis reaction in an amount of 30–50 mol%. The absence of aluminum oxide in the reaction mixture leads to the formation of SOD. It was found that the optimal temperature of the thermal treatment is 600 °C. An increase of the temperature leads to a decrease in the amount of the zeolite crystalline phase as well as in the formation of nepheline. The optimum concentration of the NaOH solution at the hydrothermal crystallization stage was defined to form both LTA and SOD. They are 2–4 mol/l for the LTA and more than 8 mol/l for the SOD. Under these conditions, the amount of the zeolites is more than 90 wt.%.