Effect of stirring on the dissolution of coal fly ash and synthesis of pure-form Na-A and -X zeolites by two-step process

Using the coal fly ash (FA), pure-form Na-A and -X zeolites were synthesized by two-step process. The FA was pretreated in aqueous NaOH solution under stirring condition at 85 °C for 18 h. The amorphous aluminosilicate of FA was dissolved during pretreatment. Increasing the stirring speed accelerated the dissolution of FA and increased Si4+ and Al3+ concentrations in the solution. This fact indicated that the stirring during pretreatment significantly affected on the dissolution of FA. After pretreatment, remaining FA was removed and aqueous NaAlO2 solution was added to the residual solution to control the molar ratio SiO2/Al2O3 of 0.5–4.5. After aging the resultant at 85 °C for 24 h, white precipitates were generated over the whole SiO2/Al2O3 range. Increment of Si4+ concentration by stirring during pretreatment increases the yield of the product. At SiO2/Al2O3 = 0.5, the material was identified as Na-A zeolite with a trace amount of hydroxysodalite. A single phase Na-A zeolite was obtained at SiO2/Al2O3 = 1.0. The Na-X zeolite was emerged at SiO2/Al2O3 ⩾ 2.0. At SiO2/Al2O3 = 4.5, a single phase Na-X zeolite was formed. The cation exchange capacity of synthetic single phase Na-A and -X zeolites was respectively 4.78 and 3.88 meq./g.