Microwave-Assisted Two-Step Process for the Synthesis of a Single-Phase Na-A Zeolite from Coal Fly Ash

Using the amorphous aluminosilicate of coal fly ash (FA), a single phase Na-A zeolite was synthesized. The FA was pretreated in NaOH solution by refluxing under microwave irradiation for 3600 s (1 h). After the pretreatment, the remaining FA was filtered off and NaOH-NaAlO2 solutions were added into the residual solution to control the SiO2/Al2O3 molar ratio ranging from 0.4 to 10.0. The precipitates thus formed were aged at 358 K for 24 h in an air oven. The amorphous aluminosilicate of FA was dissolved during the pretreatment and its dissolution rate is much faster than that using a conventional air oven. When the NaOH-NaAlO2 solution was added into the solution after the pretreatment and then aged at 358 K for 24 h, white precipitates were yielded over the whole SiO2/Al2O3 range. At SiO2/Al2O3 = 0.4–0.8, the materials formed were identified as a single-phase Na-A zeolite. The cation-exchange capacity of a single-phase Na-A zeolite formed at SiO2/Al2O3 = 0.8 was 508 cmol/kg, which is close to the 550 cmol/kg of the commercial Na-A zeolite. The hydroxysodalite and Na-X zeolite were produced at SiO2/Al2O3 ≥ 1.0.