Study of the pre-crystallization stage of hydrothermally treated amorphous aluminosilicates through the composition of the aqueous phase

A detailed study of the parameters and phases involved in the pre-crystallization of the hydrothermal conversion of coal fly ash to zeolites has been performed. The study has also been extended to pumice, a natural glassy aluminosilicate. Dissolution of the main components, their concentration ratio in the solution phase, the re-precipitation in the form of an amorphous gel from which evolves zeolite crystals, and the compositions, occurrence times and degrees of completeness of the gel and zeolite crystals have been found to be strongly interrelated and all influenced by the temperature and alkali-metal content of the treatment. In the case of coal fly ashes, the halt occurring in the conversion to zeolites some time after its beginning has been shown to be due to the mullitic phase located inside the ash particles and containing most of the total aluminium content of the ashes. In contrast to the external amorphous phase, this phase is insoluble at the experimental conditions suitable for the conversion treatment.