Microwave Assisted Two-Step Process for Rapid Synthesis of Na−A Zeolite from Coal Fly Ash

Using the amorphous aluminosilicate of coal fly ash (FA), Na−A zeolite was synthesized. The FA particles mechanically treated with ball mill were added into a tube made of cellulose filter paper, and they were pretreated with a NaOH solution by refluxing under microwave (MW) irradiation for 60 min. The amorphous aluminosilicate of FA was dissolved during the pretreatment. After pretreatment, the tube was removed and NaAlO2 solution was added to the residual solution to control the SiO2/Al2O3 molar ratio of the solution ranging from 0.5 to 4.0. The precipitates thus formed were aged by refluxing under MW irradiation for 60 min. After aging, white precipitates were yielded over the whole SiO2/Al2O3 range. At SiO2/Al2O3 = 0.5−1.0, the materials were identified as Na−A zeolite with a little amount of hydroxysodalite. The amorphous aluminosilicate emerged slightly at SiO2/Al2O3 = 2.0. The diffraction peaks of Na−A zeolite and hydroxysodalite disappeared at SiO2/Al2O3 = 4.0. The cation exchange capacity of Na−A zeolite formed at SiO2/Al2O3 = 0.5 and 1.0 was 4.32 and 4.70 meq·g-1, respectively. The results obtained led to the conclusion that Na−A zeolite can be synthesized from milled FA for 120 min under MW irradiation.