Studying Solid-Phase Processes in Metakaoline–Sodium Hydroxide Mixtures by Means of Isoconversion Analysis

Processes of the thermal treatment of 6Al₂Si₄O₇: 12NaOH mixtures for the synthesis of zeolites are studied. The mixtures are subjected to ultrasonic treatment and mechanochemical activation, after which the suspensions are evaporated, granulated, and dried. The study is performed using X-ray diffraction, synchronous thermal analysis, and electron microscopy. It is established that calcination below ~500°C leads to the dehydration of the LTA zeolite and sodium hydroaluminates formed earlier, and Al₂Si₄O₇ reacts with LTA and NaOH in the range of 500–800°C to form Na₆Al₄Si₄O₁₇ and Na₈Al₄Si₄O₁₈. Using the Ozawa–Flynn–Wall and Kissinger–Akahira–Sunose methods, the apparent activation energies (E) are calculated for this range. The two methods yield close results. It is found that E grows from 30–80 to 240–300 kJ/mol as conversion increases. It is shown that preliminary ultrasonic treatment and mechanochemical activation reduce apparent energy of activation E due to changes in the morphology of particles.