Isoconversional Kinetics of Thermal Processes in Mixtures Based on Metakaolin and Sodium Hydroxide

This study investigated the thermal behavior of the mixtures 6Al₂Si₂O₇: 12NaOH and 6Al₂Si₂O₇: 12NaOH: 2Al₂O₃ which are designed for synthesis of LTA (Linde type A) zeolite. XRD, SEM, and Synchronous thermal analysis (STA) have been used. It was found that after evaporating suspensions, molding pellets, and drying, small amounts of LTA and sodium hydroaluminates have been formed in the sample. The removal of crystallization water occurs on heating up to 400°C. In the temperature range from 400 to 850°C, Na₆Al₄Si₄O₁₇ and Na₈Al₄Si₄O₁₈ are synthesized by interaction of metakaolin with sodium hydroxide. The formation of mullite and nepheline is also observed. It was shown that preactivation of powders in the vibratory mill allows reducing the starting temperature of synthesis at 50–100°C. For the range 400–850°C using Ozawa–Flynn–Wall analysis, the values of apparent activation energy and preexponential factor have been calculated. It was established that the apparent activation energy for mixtures without preactivation made 200–290 kJ mol^?1. After preactivation, E values decreased to 130–170 kJ mol^?1. Also it was shown that alumina excess inhibits nepheline and mullite formation.