Comparison of microwave and conventional heating routes for kaolin thermal activation

The effect of activation properties of the precursors of zeolite directly prepared from kaolin influenced by microwave field and conventional heating was investigated. XRD, TG-DSC, FT-IR, SEM, particle size analysis, specific surface area (BET), pore size distribution (BJH) and N2 adsorption-desorption were discussed to determine the optimal activation temperature. It is concluded that the conversion of kaolin to metakaolin in the microwave field is at 500 °C holding for 30 min, which is 100 °C lower than that in conventional calcination and 90 min shorter, and the phase transition process of kaolin under the effect of microwave field is the same as that of conventional heating method. SEM analysis indicates that the particle size is more uniform and agglomeration appears slightly in the microwave field. The N2 adsorption-desorption isotherm, BET and BJH of kaolin indicate that the pore properties are almost invariable regardless of calcination route during the process of calcining kaolin into metakaolin. It indicates that microwave calcination is superior to conventional calcination in the activation pathway of kaolin. It is attributed to microwave heating relying on objects to absorb microwave energy and convert it into thermal energy, which can simultaneously and uniformly heat the entire substance.