Salt-thermal zeolitization of fly ash

The molten-salt method has been recently proposed as a new approach to zeol itization of fly ash. Unlike the hydrothermal method, this method employs s alt mixtures as the reaction medium without any addition of water. In this study, systematic investigation has been conducted on zeolitization of fly ash in a NaOH-NaNO3 system in order to elucidate the mechanism of zeolite f ormation and to achieve its optimization. Zeolitization of fly ash was cond ucted by thermally treating a powder mixture of fly ash, NaOH, and NaNO3. Z eolitization of fly ash took place above 200 degreesC, a temperature lower than the melting points of salt and base in the NaOH-NaNO3 system. However, it was uncertain whether the reactions took place in a local molten state or in a solid state. Therefore, the proposed method is renamed the "salt-th ermal" method rather than the "molten-salt" method. Mainly because of diffi culty in mobility of components in salt mixtures, zeolitization seems to oc cur within a local reaction system. In situ rearrangement of activated comp onents seems to lead to zeolite formation. Particle growth, rather than cry stal growth through agglomeration, resulted in no distinct morphologies of zeolite phases. Following are the optimal zeolitization conditions of the s alt-thermal method: temperature, 250-350 degreesC; time, 3-12 h; weight rat io of NaOH/NaNO3, 0.3-0.5; weight ratio of NaNO3/ fly ash, 0.7-1.4. Therefo re, it is clear from this work that the salt-thermal method could be applie d to massive zeolitization of fly ash as a new alternative method for recyc ling this waste.