Utilization of NaCl for phillipsite synthesis from fly ash by hydrothermal treatment with microwave heating

The coal fly ash was treated hydrothermally with the mixture of NaOH aqueous solution and NaCl aqueous solution at 373 K, using the microwave heating and the conventional heating in order to clarify influences of the NaCl concentration in the hydrothermal solution on the growth rate and the crystalline phase of synthesized zeolites. As a result, it was found that phillipsite crystallinity of product powder increases with the additive NaCl concentration in the hydrothermal solution when the concentration of NaOH is so low as not to generate the zeolite from fly ash. On the other hand, in case that NaOH concentration is relatively high, the addition of NaCl to the hydrothermal solution using the conventional heating hardly affects phillipsite crystallinity of product powder; however, the generation of hydroxysodalite is enhanced by the microwave heating. Moreover, it was revealed that the necessary treatment time to complete the crystallization of phillipsite increases with increasing concentration of NaCl in the hydrothermal solution. This is because the substitution of NaCl for NaOH reduces the dissolution rate of aluminate and silicate ions from fly ash and the generation rate of the precursor aluminosilicate gel. However, this delay of the completion of the crystallization can be shortened by the microwave heating method. These experimental results concluded that the utilization of NaCl for the hydrothermal solution is effective in phillipsite synthesis from fly ash only when the NaOH concentration is relatively low in the hydrothermal solution and especially, when the microwave heating is used.