Synthesis of granulated binder-free LTA zeolite from metakaolin using ultrasonic treatment

Abstract

It was studied the influence ultrasonic treatment (22 kHz) of an aqueous suspension of metakaolin on the synthesis of the granulated binder-free LTA zeolite. Mixtures with a molar ratio of the solid ingredients of 6Al2Si2O7:12NaOH (stoichiometric) and 6Al2Si2O7:12NaOH:2Al2O3 have been investigated. XRD, SEM, FT-IR and rotational viscosimetry have been used for testing. It was established that in the mixture with an Al2O3 excess after the ultrasonic treatment, sodium aluminates of cubic syngony were synthesized. Moulding pastes have been prepared from the sonicated suspensions and pellets of 3 mm diameter have been formed by extrusion. It is shown that sodium aluminates increased both the strength of coagulation structure of pastes and the mechanical strength of the pellets. It was found that after the thermal treatment of the granules at 650 °C, Na6Al4Si4O17 was formed. In the mixture with an Al2O3 excess, Na8Al4Si4O18 was also detected. The zeolitization of sodium aluminosilicates and unreacted metakaolin during hydrothermal crystallization in the NaOH solution with concentration of 2 mol l−1 allowed synthesizing the LTA zeolite. The pellets from the stoichiometric mixture contained 91 wt% zeolite and the remainder was unreacted Na6Al4Si4O17 and other byproducts. The pellets from the mixture with an Al2O3 excess contained 97 wt% LTA.