Reaction, mechanism and application of various zeolite syntheses from coal fly ash

The hydrothermal syntheses of zeolites from coal fly ash were carried out using NaOH and KOH as an alkali source in order to clarify the reaction mechanism of zeolitization. The physical properties such as surface structure, crystal structure and cation exchange capacity (CEC), and the ion exchange properties were measured for the zeolites synthesized by hydrothermal treatment. From these results, the effective usage was investigated for various zeolites derived from coal fly ash. Zeolite P, HS (hydroxysodalite) and K-CHA (potassium-chabazite) are mainly formed as zeolite species in NaOH or KOH solution at 393 K. The zeolite P and K-CHA have a large exchange capacity as a cation exchanger. There are three reaction steps; the dissolution, deposition and crystallization reactions in a series of conversion process from coal fly ash to zeolitic materials. The selectivity of cation exchange of various zeolites is explained by the Coulomb's attractive force between the ionic charge of metal ions and the surface charge of zeolites. The obtained zeolites can be used as a cation exchanger and a soil improvement agent. The usage of the synthesized zeolites as soil improvement makes a contribution to return coal residue to soil environmentally friendly.