Formation process of Na-X zeolites from coal fly ash

In order to synthesize Na-X zeolite from coal fly ash (Fa), Fa was pretreated under stirring condition at various temperatures of 20–50°C for 72 h and then aged at 85°C for a given period with NaOH solutions. The resulting materials were characterized by various means. When Fa was aged for 72 h without pretreatment, species P were formed. As the pretreating temperature raised from 20 to 50°C, the degree of crystallinity of faujasite increased, while that of species P decreased. The faujasite species formed was identified as Na-X zeolite with molar ratio SiO₂/Al₂O₃ = 2.4. When Fa was pretreated at 50°C and aged for 60 h, the only species formed was Na-X zeolite. Increasing the pretreating temperature up to 50°C results in the increase of Si⁴⁺ and Al³⁺ concentrations in the treating solution by dissolution of amorphous material in Fa. With the conditions used, the crystalline phase, such as -quartz and mullite, was poorly dissolved during the treatment. Hence, the higher pretreating temperature would give the uniform nucleation and crystal growth of Na-X zeolite during the aging.