The effect of alkali concentration and solid/liquid ratio on the hydrothermal synthesis of zeolite NaA from natural kaolinite

Zeolite NaA and hydroxy sodalite materials were synthesized successfully by hydrothermal method using the natural kaolinite. In the first step, kaolinite has been calcined at 600 °C for 2 h. IR spectra and DTA-TG analysis of the calcined kaolinite at 600 °C confirm the transformation to the metakaolinite phase which has been detected during the calcination process. As a second step, the zeolitisation experiments have been carried out under hydrothermal conditions. The metakaolinite obtained has been reacted with NaOH solutions of different concentrations in autoclaves at 105 °C for 2 h. The reaction product was found to be essentially zeolite NaA with the NaOH solution of 4 N. The effect of solid/liquid ratio on the reaction products has also been investigated. Zeolite NaA was the main product with solid/liquid ratios of 1.25-5.00 g/25 ml after which hydroxy sodalite was observed to be formed in the reaction mixture. The products obtained from the experiments have been characterized by using X-ray diffraction (XRD), FTIR and DTA/TGA methods. Zeolite NaA can be obtained from kaolinite under the conditions applied showing that a metakaolinisation can be achieved at 600 °C which is much lower than the temperatures given in the literature, 700-900 °C and also the time necessary for formation of zeolite NaA is less than that of the previous works.