A comparative study using two methods to produce zeolites from fly ash

Two methods have been evaluated for the synthesis of zeolites using a UK fly ash as raw material. One method consists of a conventional hydrothermal technique in which fly ash is mixed with NaOH solution and the other comprises a combination of alkaline fusion of fly ash with NaOH prior to a hydrothermal treatment, where the fusion product is mixed with water. Both processes have been evaluated at different conditions of temperature, time and proportion of NaOH:PFA:H2O in order to optimise the synthesis. Products have been characterised in terms of mineralogical composition and cation exchange capacity. Experimental results have demonstrated that parameters such as the NaOH/PFA ratio, time and temperature have a significant effect on the type and degree of zeolitisation achieved, with types X being dominant under optimised conditions. In both cases, the synthetic zeolite products display significantly increased adsorption capacities compared to the raw material. However the fusion method yield better results in terms of shorter times necessary to obtain a product with high crystallinity and excellent performance as cation exchanger. (C) 2003 Published by Elsevier Ltd.