Two-step process for synthesis of a single phase Na–A zeolite from coal fly ash by dialysis

Using the amorphous aluminosilicate in coal fly ash (FA), a single phase Na–A zeolite was synthesized from FA by dialysis. The FA and NaOH solution added into the tube made by semipermeable membrane were pretreated in the same NaOH solution at 85 °C for 24 h. After the pretreatment, the tube was removed and NaOH–NaAlO2 solution was added into the residual solution to control SiO2/Al2O3 molar ratio of the solutions from 0.9 to 4.3. The precipitates thus formed were aged for 24 h at 85 °C. The amorphous aluminosilicate in FA was dissolved during the pretreatment. When the NaOH–NaAlO2 solution was added into the solution after the pretreatment and then aged, white precipitates were yielded over the whole SiO2/Al2O3 range. At SiO2/Al2O3=0.9, the material formed was identified as a single phase Na–A zeolite. The Na–X zeolite was slightly produced at SiO2/Al2O3≥1.7.